Why can't we get mastery tokens in ARAM, Twisted Treeline and Rotating Game Modes?

I know you get a token for playing overally well on Summoners Rift (so vision control, objectives etc. included). But let's be honest, the first thing that comes to your mind when you see someone with level 6 or 7 is that they're just mechanically good with that champion. And that's not even entirely true, because if you play a champion over and over on Summoners Rift, eventually you'll reach level 7 even if you're pretty bad. Meanwhile, if you play a champion perfectly on TT or ARAM, you're stuck at level 5 forever. And champion mastery is just cosmetics anyway. So I see no reason why level 6 and 7 shouldn't be achievable outside of Summoners Rift.