I disagree that most apps are CRUD. To the contrary, most apps starting out as naive CRUD have complex implicit constraints related to eg. in what state you can modify resources in a particular way.
Even for CRUD apps with simple master/detail data relationships it doesn't make sense to tie your domain design to network requests.
So by saying "we're going to go with REST", you can determine about 90% of your API design more or less instantly, and it rarely gets in the way.
You still have to get your domain modeling right, and sometimes you need to make some extra resources. But you have a design that, basically, works.