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.
Not talking about equipment - there are several books you can read to gain a permanent +1 to an attribute, there are magic beans you can plant and eat the fruit from with a chance to either blow up (10d8 force) or gain a permanent increase to an attribute just to name a few.
