because it's dumb..
Originally this was done to cater to the Lubing fad..
But cherry mx is not designed to be lubed, the lube simply just rolls off, it will not stay on there for long. and because the switch has such large openings any lube greatly increases the odds of dust causing build up on the inside.
Then people were like, oh well, now we're gonna swap the stems..
Yea ok, have fun spending 2 hours doing that.. I'd rather go to work for 2 hours and buy a new board in another switch variation..
Swappable switch plate IS versatile, but it's just not practical in usage...