Actually this is wrong. Yes the constant WP_POST_REVISIONS exists. But WP Engine has disabled this constant. They do in fact "butcher" WordPress in the sense that they remove a feature, you can't turn it back on by yourself, and you need to talk to their support to get a limited version of it re-enabled by them.
Add to that, revisions are a big deal for a certain type of customer. Say, an enterprise scale publisher for example who has built an extensive publishing workflow around WP. (Hacker News consistently underestimates how massive some WordPress installs are; the scale of the world's biggest publishers, many of whom rely on WordPress, blows your nice little startup out of the water.)
So I am aware of a case or two where this actually became a negotiating point in an enterprise contract. The customer was entirely unaware that revisions were just a free thing built into WP, and it influenced the resulting contract and cost. Dirty dirty on WPE's side, really.
BTW this is documented at https://wpengine.com/support/platform-settings/ and you can see on that page that they limit their environment in many other ways. In the abstract this may not be a huge problem, hosts have costs and security and various limitations to think about. In my personal experience there are limitations which are not listed on that page and those are more frustrating.