Why do tanks only go to top? I feel like it should vary depending on the lineup, like why not send 2 ranged dps to top so they can harass the tank and have an even tankier dude go bot so he can leech xp.
Support Tanks will still end up on the squishy side due to reduced gold income, unless their name is Alistar.
Jungle Tanks means giving up on a potential playmaking jungler.
So yeah, in the current meta tanks will usually end up either top or support but top lane is populated by anything a team needs. Including but not limited to fighters, tanks and mages.
Why only send AP mid? Wouldn't really any hero work in lane since there's no river height advantage and no runes.
Anything except tanks works just fine. In the current meta, you'll get punished heavily for running a mid laner without strong ranged waveclear, something most melees, assassins and physical damage dealers lack.
Since junglers exist why have mid work as a ganker? Seems like the guy who walks around all over the place would be the one ganking heroes instead of the guy who would leave the lane completely uncontested.
Why limit yourself to a single roamer when you can have three (Jungler/Mid/Support)? Strong waveclear and proper rotations should be enough to keep your mid turret defended. Don't forget Top laners with Teleport can also join in the fun too.