I think freakin' Wii Sports had the option to change the match length, but it's been so long since I last played it that I can't remember exactly. It's indeed odd that this feature has evaded Mario Tennis Aces for so long, but imagine there was a time when you couldn't even directly select a court or restart a mission from the pause screen. At least adding new characters made sense, but what kind of favour did omitting quality of life features bring them??
Regardless, I feel like the game deserves to be talked about a bit more often. It now has a fan-favourite roster with additional character skins (though yeah I also miss Baby Mario and Baby Luigi), looks nice, and plays good... if you don't have drift in your Joy-Cons, that is. There should be a thread dedicated to that.