Not the downvoter but SBMM is a necessary evil if you have a matchmaking system instead of purely community servers.

In community servers, there's an equal emphasis in the community as well as the gameplay, so you don't mind getting dunked on as long as you have some fun with your teammates and dick around in chat.

When there's matchmaking, however, everyone aims to win the match and move on. If you don't have some type of SBMM, the new guys just get dunked on over and over with little chance to learn.

The example I have for this is TF2. In Valve servers the players with thousands of hours completely dominate everyone else, and without a sense of community this got tiring for me pretty fast, even on TF2 where there are some impromptu fun moments (conga chains etc). I tried to play circa 2015, I think they added some sort of sbmm afterwards with the competitive updtae.

Note that I say this as an argument in favor of community servers, not in favor of SBMMs, but as long as corpos make the games that's not gonna happen sadly.

You asked how SBMM is good, so I'm gonna write way too much to answer why I think that

SBMM is important for games with matchmaking because competitive games are generally both more fun and more fair when players are matched with other players around their current skill level, people generally don't like when a game is just rigged from the start and they can't contribute anything because of it

It's pretty bad for people learning and improving too to not have it, because it's a lot harder to learn from a loss if you just get blown out immediately by someone so far above your level, as opposed to someone closer to your skill level where it's easier to tell what they're doing differently compared to you

It's also a weird complaint to say that the game "punishes" you for sub-optimal behavior and playing casually, because what really happens is that you maybe lose a few matches before the game realizes you're playing worse and puts you around that level, meanwhile if you don't have SBMM you will always have a pretty good chance of being matched against someone trying as hard as they can to win and optimizing their gameplay and equipment (this has been a common complaint about Destiny when they have removed SBMM in PvP in multiple instances, because it lead to it being basically impossible to play casually because basically every game is full of pubstompers)

If CoD's implementation doesn't work like I described, then that's a CoD problem, not an SBMM problem, there are loads of games with SBMM where people are generally happy with it (like Splatoon, or most fighting games)

It's also worth pointing out that despite a lot of people thinking SBMM in CoD is some recent thing, it's actually been a thing for most of its existence, same with early Halo

And as far as being casual and "chilling out and having fun" goes, you also have to keep in mind that you don't actually know how seriously your opponents are playing at any given moment, it could very well be the case that they're also not taking it too seriously and just seem like they're "tryhards" because they happen to be a lot better, a problem that's improved by SBMM rather than made worse because you're ideally (as in, in games that do it well, not as sure about CoD) less likely to go against people like that if you're not actually on their level