I suspect it's a deliberate choice both as a fashion statement and to discourage repair.
It's interesting to see a similar phenomenon with desktop PC motherboards: "gamer" or otherwise "enthusiast" boards are often a non-green colour and come with superfluous design elements (like plastic covers over the actual components, which sort of remind me of the engine covers on newer cars...), while both the ultra-cheap prebuilt and ultra-expensive server boards remain green and free of fluff.
I doubt it has anything to do with repair. When you’ve got a 18 or so layer PCB, probably with microvias (invisible if they’re in pads), and very closely spaced QFN and BGA-packaged parts everywhere, soldermask colour is the least of your problems. So much of the design is on internal layers inside the board and so much interconnection to internal layers in pads (if using microvias) and under BGAs that being able to easily see the top and bottom layer traces easily would give you very little to go on. You’d really need design files to do anything more than just reflowing the odd part...
It probably is a lot to do with fashion though. It’s literally one of the only reasons that I always specify blue or black soldermask at work - just looks a lot cooler. Green boards just have that late-90’s electronics look.
I blame case windows for this. The last desktop I built, it was a chore finding a case that didn't look stupid, and I eventually gave up on a normal motherboard, as anything with a z170 chip set looked like it belonged in a transformers toy.
And while people definitely geek out over servers, it's nearly all about specs. After all, why make something that gets hidden away in a rack in a DC look pretty?
I suspect that is one reason Apple isn't using green mask, since the connotation for many people is 'cheap'
It's interesting to see a similar phenomenon with desktop PC motherboards: "gamer" or otherwise "enthusiast" boards are often a non-green colour and come with superfluous design elements (like plastic covers over the actual components, which sort of remind me of the engine covers on newer cars...), while both the ultra-cheap prebuilt and ultra-expensive server boards remain green and free of fluff.