It is because people are not one dimensional. Most people have talent for range of professions, they are not predisposed to like exactly one of them. Also, most good students will end up liking whatever they picked provided it is within range of things they are not completely failing at.
Learning something often makes one like it more, even if one was initially ambivalent. Plus, once you know a bit more about field, you can find sub-field you are actually good at and find fun. As in, various tech jobs require slightly different aptitudes - some require more patience, some more abstract thinking, others memory. But, you dont know until you try.
You're only going to get a good TA when that professor is teaching a class that's their focus and has a grad student that has that fucus AND that student isn't on other research funding. The system is far from optional for either undergrads or grads. Believe me, I'm not happy either
I did not find the boom period as bad as I expected. Many of the students in it for the money were still very smart and motivated and interesting to talk to. But I preferred the non-boom periods nonetheless :)