That's because some people don't like the stress of Big Law so feel in-house is less stressful, people want to try new things etc. People also get told by their firm to leave (if they're not going to make partner or they're not billing enough etc) so the next port of call is in-house. I know this is particularly the case for Kirkland where they make you a non-equity partner but if you can't build a book of business, they tell you to leave (and help you land in-house roles).
The billables model is also very difficult for some people. If you're not billing, it can be stressful and the allocation of work isn't always even. Partners/senior associates have favourites and so if you're not being given work, this can also be stressful. But I think this is more of a problem for junior lawyers in certain practices.
Most careers don't have a billables model where you have to seek work otherwise you're not going to have a job for long. I've been in a corporate role at investment firm where I used to go watch Netflix in between when I wasn't being given work and nobody cared as long as I indicated I had capacity. In a billables model, I don't think I could get away with that for long.
