There are quite a few mouse and keyboard adapters out there. Go read about the Rainbow Six ranked mode on consoles that is dominated by mouse and keyboard players. It forced all the console players to switch to PC to be on equal footing.
That said, there are really good players out there as well. My friend's son gets accused of hacking all the time, and he's using nothing but a regular Xbox controller. The kid is inhumanly fast and is 100% legit. On top of that, at any given moment, someone can get lucky. I was playing Destiny 2 and 180 no-scope sniped some guy while I was in mid air. I'm sure that guy was like WTF, but don't ask me to repeat that again. I don't even regularly use snipers in that game.
The problem is that the skill based matchmaking is not very good. It really just overreacts to your last 3-5 games. Many of us end up bouncing between very different skill brackets every 3-5 games. I spend 3 games going positive, playing against people who hit some shots and miss others - people at my skill level or even a little below. Then after a couple games positive, it is time for me to get crushed in these hardcore lobbies. Players spinning jump headshots around corners. I'm pre-aiming a corner and they jump around and headshot me. They are insanely good. This is where the SBMM fails for me, because clearly I'm well out of my skill bracket. Then 3-5 games later, the process repeats. I drop back down to my skill level and have lots of fun.
This is why it feels like people are cheating too. You go from a normal lobby to a top tier one without warning. You have no idea that you moved until you are in game and suddenly 0-5, barely getting out of the spawn.
Real SBMM systems look at your overall performance and also factor in your recent games. COD just looks at the latter, which is why it feels so off. Destiny 2 and lots of other games have SBMM, but it never feels this severe.
The broken SBMM is the reason I mostly don't play anymore. It is like playing the AI in an EA Sports game. Do well and suddenly the tables turn against you.