Well, there are a few of reasons I can think of ...
- Gravity just works better. You can get more oil out of the sump using gravity than you can with a pump. Albeit, in some cases what's left by the pump may be negligible.
- Gravity is easier. You just let it do its work. Pull the plug and let physics do its thing.
- Gravity doesn't take as long. You don't have to stand there and keep working the pump, which frees you up to do other things, like prep the new oil filter.
- You don't have to purchase an extra item (the pump) to make things happen. You still have to have some container to keep the old oil in until you use the new oil (and thus have an empty container).
Most garages will have lifts, which makes getting to the oil plug all the more easy.
If one of your arguments for using a suction pump is that you don't have to jack up the car by using one, most cars' oil filters are located underneath, so you have to jack it up, put the pan under there to catch any dripping oil, then crawl under and remove/replace the oil filter.
As for your last question, depending on what it is, if you have part of the drain tube drop down into the sump, you can possibly get it out through the drain plug hole, but worst case, you have to pull the oil pan (sump) in order to retrieve it. I wouldn't want that left in my sump. And think of the time it takes to pull the sump? When you're talking time, you're talking money. Not wasting either is quintessential to operating a prosperous business.
Don't get me wrong, some people swear by using a suction pump to get the oil out. In some cars it's just easier to use one all around, due to their being shrouding underneath the vehicle which has to be removed. There are reasons to use one. Most shops just aren't going to spend the time/money to get/use one when they really aren't more efficient than just using gravity to do its thing.