20 is only the cap from increases from Feats or Ability Score increases tied to leveling up, there are plenty of items in 5e (and previous editions) that allow you to increase it further
I don't count equipment as part of your basic stats.
Also 15 is not the cap when rolling, it's only the cap when using Point Buy or Standard Array.
Well, that's... pretty fucking stupid, then.
Now I think I dislike the idea of rolling for stats even more.