- Because they read it closely but didn't interpret it correctly.
- Because they brought it up and their counterpart said "don't worry, that's boilerplate and we've never actually used that clause".
- Because the alternative was signing a contract that had no relocation/hiring bonus.
- Because they needed a job, and that's what was there.
- Because they really really needed a job to keep health insurance or their visa, and they had no leverage.
- Because they had leverage, but were too inexperienced in negotiations to realize it.
- Because they had leverage, but were not comfortable using it, and were taken advantage of.
There's lots of reasons, of varying quality, that someone may do something that seems ridiculous to you. Why are you implying that the (hypothetical) losing party here is the one who's done something wrong?