I believe it is because...I agree that pursuing max 20s isn't really that big of a deal, but I don't see the benefit to the game in leaving that option in the game at all. Why create a tension between pursuing mathematical effectiveness and choosing fun feats to develop your character when it isn't necessary?
Either cap stats to 18, or raise the values of the starting array/point buy, or make ASIs so they can't bump a stat by 2 (either get rid of it or make it give 2 +1s.)
1) A lot of people don't actually see it as a tension.
2) A lot of people do not want to use feats, or use as little feats as possible.
3) A lot of people don't really care whether you bump ability scores by higher point buy or ASI feats-- an increase is an increase regardless of how/when you get it-- so no method is necessarily better than the other where we need to eliminate one of them.
4) A lot of people don't even consider feats to be "fun" in the first place, because they are just additional game mechanics to have to think about-- one additional mechanic in a pile of 30 for your character-- so having it or not having it does not "fun" make. So making sure all players have access to them is not an actual benefit across the board.
WotC's chosen this path in their game-- ASI or feat at 4th, 8th, 12th level etc. And there's no substantive reason given yet to change that-- either balance-wise or fun-wise. While some players want (or feel the need for) both, and some players want (or feel the need for) neither... those players do not make up a substantial amount of the playerbase to warrant a wholesale change of the system. Those players either just have to accept what is given, OR add in their own house rules to double up / remove them at their own table.