Because that's hard to review.

I am doing a lot of exploratory programming and way point commits.

I don't need the reviewer to understand all the mistakes I made and bad designs I considered. It's enough work to understand the finished design.

As a rule of thumb, every commit that lands in master's history should build and pass tests. So that eg 'git bisect' works.

But it's not a good idea to put that same requirement on waypoint commits I make along the way, when exploring.

Because then you're going to merge in the ugly commits and make everyone who needs to look at the history in the future have to work that much harder to understand what's going on.