Because in big commercial projects several people or teams merge inside the main branch on a regular basis. It's easier to look at a history only including squashed commits each encompassing an entire PR (feature or a bug).

It gives you better high-level observability and a good bird's-eye view. And again -- if you need more details you can go and check all separate commits in the PR/branch anyway.

And finally, squashed commits are kind of atomic commits. Imagine history of three separate PRs happening at roughly the same time. And now all those commits are interspersed in the history of the main branch.

How is that useful or informative? It's chaos.

EDIT: my bad, I conflated merging with rebasing. Still, I prefer a single squashed commit for most of the reasons above, plus those of the other two commenters (useful git blame output and buildable history).