It's because it actually is a lot of money for academia. The "make 200k-400k signing bonus at a tech firm" is approximately the same as "just have the truck drivers join FAANG for a 50k signing bonus" - as in, there aren't that many positions that pay that well, there's a ton of competition for them, there isn't enough for everybody, so the vast majority of people are going to take this option for a variety of reasons.