The only reason I can see for that is that they prepare for a deprecation of crossing=*, which is currently prevented by fears about backwards compatibility.
I also blame StreetComplete a bit for that, they don’t want to add a question about the kind of marking before someone can’t proof them that there are actually users that tread some values differently then yes and in which countries markings have which legal meanings. Right now they just add crossing:markings=yes on everything since the editor already abandoned crossing=*
But putting that list together and convincing them is an other project than this one
The fact that iD cuurently has no preset for a zebra crossing and just has crossing:markings=yes in every preset doesn’t help either. But fixing this was also one point of the proposal, to agree on using crossing:markings=zebra for newly mapped ones. After they burned themselves quite a bit on their last attempts at making a preset for that it doesn’t look like they want to create a new one without a structured community approval.