That's... the standard 5th edition system?
Stats (pardon, "abilities") without racial modifications are capped at 15, even if you roll for them. And then they can be increased to a max of 20 over time levelling up.
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 - 30 is the hard cap.
Also 15 is not the cap when rolling, it's only the cap when using Point Buy or Standard Array.
