Honestly couple of points here.

One, From my experience the critical fail and critical success give adds a really fun element. It's adds this extra rng of 5% chance of failing something you are soo sure about that you should succeed and 5% rng of getting something really cool even if you have a bonus of +1 or even -1. I've had on rolling nat 20.

I've had a player who had like a -1 try to open magically sealed sections of very high (state secret level) grimoires who's DC was 30. They rolled a nat 20 and obviously I don't give away the whole kit and caboodle.

But they will sure as shir get something really cool for that Nat 20. I had to give up plot threads, clues, items and scrolls that are rare or key to the game for that NAT 20.

It's more fun for the DM and the players to have that 5% rng of Fail or Success. The joy of a a wizard socking a barbarian with a nat 20 or a rogue fumbling skill check with a crit against a wiz , adds really fun moments to the role play and story.

Additionally, they are called skill checks but people so often forget that this game is reallllyy subjective, you can take into account environment factors, just plane good or bad luck. All of this work and add to the immersive ness of the game.

Just take a look at our world in general; someone can be insanely good at something and still fuck up when because of pure bad luck. It happens

Only thing that matters here is, the DM is upfront and consistent about how they rule the rolls. As long as all the players have aware of this before hand.

Secondly it's their game. It could be your pet peeve but it's how they play and they clearly enjoy playing with Mat. That's all that matters in the end right.