NOTE: This answer is put here to add to Steve Matthews answer, as he has given some very compelling reasons.

I want to talk to the three points you've added:

These (and likely more) techniques, have additional practical benefits. Three immediately come to mind:

 

1. No runs or drips.

While you are right there's no "runs, drips, or errors" with electroplating, car manufacturers have spray painting down to a fine art with regards to vehicles coming off the assembly line. If I were to wager a guess, over 90% of the cars produced in the US are painted on an assembly line where robots are used to ensure accurate painting. Here there are so few "runs, drips, or errors", it's almost not worth mentioning. The robots are that good.

2. You can't paint what you can't reach. Want to paint the cavity between two sheets of metal (perhaps a bonnet/hood or trunk/boot-lid) so they don't rust again? Good luck with that.

You don't need to paint between layers, because primer/sealer will do the job for you. Again, many cars today use a dipping process where the entire body is dunked, then flipped within a bath of primer/sealer. This ensures COMPLETE coverage of all surfaces to prevent rust and corrosion. This process is good enough for the manufacturers to warranty rust through of body surfaces for many years after the automobile is put together.

3. Plastic (i.e. painted) barriers are likely around three orders of magnitude thicker than electrolytically deposited barriers. What? Consider the difference between a metre and a kilometre. That's three orders of magnitude, and the relative difference between microns & millimetres. And then? Paint a bolt. The whole thing. Yes, including the thread. Primer, several coats, and clear. Now do the nut. When they're ready, try uniting them without stripping the paint. You can't. In many cases (particularly with fine threads), the thread won't really resemble much of a thread at all; each layer brings the valleys closer to the peaks, and tolerance is exceeded.

Most bolts don't get painted, unless it's the head of the bolt (because it is holding body panels together before and after the painting process). There's no real need to. If there is a specific need for a bolt to need corrosion resistance, they will zinc plate it or put some other form of corrosion inhibitor onto the fixture. Most nuts/bolts don't need it though. Even then, most manufacturers don't worry about it.

There are two things you need to remember here.

1. Exactly what Solar Mike said in the comments: "two words : speed & cost" ... The manufacturers are trying to produce the vehicles as inexpensive as possible. The more it costs them to produce, the more they pass onto the consumer. And no matter where you go, time is money.
2. Cars aren't made to last forever. Manufacturers would be stupid to even attempt it. Their whole game plan is for the car you buy to last you for several years at which point you'll come back and purchase a new one from them. If they made cars which lasted forever, you'd never have to come back. Besides, nobody that I'm aware of has produced a perfect machine (of any sort) which doesn't wear out of some kind of time period. They just don't exist.

NOTE: This answer is put here to add to Steve Matthews answer, as he has given some very compelling reasons.

I want to talk to the three points you've added:

These (and likely more) techniques, have additional practical benefits. Three immediately come to mind:

1. No runs or drips.

While you are right there's no "runs, drips, or errors" with electroplating, car manufacturers have spray painting down to a fine art with regards to vehicles coming off the assembly line. If I were to wager a guess, over 90% of the cars produced in the US are painted on an assembly line where robots are used to ensure accurate painting. Here there are so few "runs, drips, or errors", it's almost not worth mentioning. The robots are that good.

2. You can't paint what you can't reach. Want to paint the cavity between two sheets of metal (perhaps a bonnet/hood or trunk/boot-lid) so they don't rust again? Good luck with that.

You don't need to paint between layers, because primer/sealer will do the job for you. Again, many cars today use a dipping process where the entire body is dunked, then flipped within a bath of primer/sealer. This ensures COMPLETE coverage of all surfaces to prevent rust and corrosion. This process is good enough for the manufacturers to warranty rust through of body surfaces for many years after the automobile is put together.

3. Plastic (i.e. painted) barriers are likely around three orders of magnitude thicker than electrolytically deposited barriers. What? Consider the difference between a metre and a kilometre. That's three orders of magnitude, and the relative difference between microns & millimetres. And then? Paint a bolt. The whole thing. Yes, including the thread. Primer, several coats, and clear. Now do the nut. When they're ready, try uniting them without stripping the paint. You can't. In many cases (particularly with fine threads), the thread won't really resemble much of a thread at all; each layer brings the valleys closer to the peaks, and tolerance is exceeded.

Most bolts don't get painted, unless it's the head of the bolt (because it is holding body panels together before and after the painting process). There's no real need to. If there is a specific need for a bolt to need corrosion resistance, they will zinc plate it or put some other form of corrosion inhibitor onto the fixture. Most nuts/bolts don't need it though. Even then, most manufacturers don't worry about it.

There are two things you need to remember here.

1. Exactly what Solar Mike said in the comments: "two words : speed & cost" ... The manufacturers are trying to produce the vehicles as inexpensive as possible. The more it costs them to produce, the more they pass onto the consumer. And no matter where you go, time is money.
2. Cars aren't made to last forever. Manufacturers would be stupid to even attempt it. Their whole game plan is for the car you buy to last you for several years at which point you'll come back and purchase a new one from them. If they made cars which lasted forever, you'd never have to come back. Besides, nobody that I'm aware of has produced a perfect machine (of any sort) which doesn't wear out of some kind of time period. They just don't exist.