Because then you don't see all those minor little changes and can view the PR holistically. I don't typically find that viewing the changes in chronologically order buys much, it's not the order I care about, it's the final changes themselves that have an impact on the system, especially when those changes could be transient (changing something in commit #1, change it back in commit #3). There's a larger load to comprehension with a squashed commit, but it shouldn't be THAT large, and it removes all the temporal noise.
It also makes tracing the historical "why" of a change easier to understand. If you don't squash it's a random change in the log, if you do squash then it's "adding feature x".
