Lead-acid batteries are rechargeable. Have you tried recharging it? That often will do.
Lead-acid batteries fail after 4-7 years
The simplest reason is there's acid in them. But some other battery types don't do any better, look at any cell phone battery.
Lead-acid batteries hate deep cycling
If you draw out most of the power of a battery, and then recharge it, that is called a "deep cycle". This is different from starting a car normally, where you take a small amount of energy "off the top" for cranking, and then quickly charge it back up once the alternator is running.
A unique curse of the lead-acid chemistry is that deep-cycles damage the battery. Other battery types don't have that problem. Deep-cycle-rated lead-acids, like Optima yellowtops, do better -- but they still will be destroyed by deep cycling, just will get more cycles before they do (on the upper range of the numbers I'm about to give).
- If you drain it dead, you'll get 5-30 cycles before battery death
- If you drain to 50%, maybe 20-200 cycles
- if only 25% depth-of-discharge (using 25% of capacity) many hundreds of cycles
- 15% DOD thousands of cycles.
If you design with batteries, e.g. off-grid solar systems, nobody will tell you this, it's just something you're expected to know... SMH...
How does that happen in a car? When you forget and leave lights on, or have a wiring fault which drains the battery, and find the car dead and jumpstart it or put it on a battery charger to get it going again... and that becomes a habit.
They don't like being left discharged, either
Another thing that kills car batteries is leaving them in a state of discharge for awhile. Suppose the battery gets drained and you just store it drained instead of recharging it. Or if the car is parked for months and has an electrical drain (as some modern cars do). Lead-acid batteries must be stored at 100% charge.
Another killer is winter cold plus discharge. Since the chemical reaction is lead vs acid, at 100% charge, the acid is very strong. At full discharge, the acid is very weak. Strong acid freezes well below minus 40 degrees. Weak acid is mostly water and is much more vulnerable to freezing at common winter temperatures. When it freezes, it cracks the battery's case, and it's done.
Surely there must be a better battery
Once upon a time, there were a variety of batteries on the market, and all of them are fine with deep discharge.
The famous "Edison Cell" was made for electric cars, is nickel-iron, and is virtually immune to abuse, having none of the above problems, and last 40+ years. It's not good at huge amounts of surge current, though, that's the one thing lead-acids are better at.
It evolved into the nickel-cadmium, either in a traditional wet-cell or a sealed AA or D-cell. They solved the surge current problem, in fact they can start jet engines - that's how jetliner APUs start. (and in the linked video, that's a really old battery.) These batteries last 20 years too. Of course they're more expensive.
Now, lithium batteries are literally exploding onto the battery marketplace. Every cell phone, most laptops, and the Boeing Dreamliner use them, and they could make a viable car battery. However nothing indicates they'll last any longer than lead-acids.