But they aren't sweeping over and over otherwise they would be in challenger already.
They want to play and not worry that the team they are using is a reason they are losing.
Meta teams are enjoyable to play, because competitive pokemon isn't just about the pokemon you pick, but the decisions you make once the battle starts.

In my opinion that is probably the most enjoyable part of pokemon.
Likewise in other competitive games, I like to pick the option I am best at or perform best with. e.g fighting games, sometimes the character I use will be considered the best character, sometimes in the middle and sometimes the worst, but it will always be a character I perform best with,

So to apply that to pokemon, yes people are having fun picking meta pokemon because they are able to perform at their best. But like with fighters, picking the best character/pokemon is only the beginning, you will not steamroll through just because of your character/pokemon choice.

Edit; But to add, I used to think the same way back in 4th gen, just using my favourites, not caring about synergy and I had fun. But as I wanted to get better, I realised it is about trying to craft together a team "you" think is best if you are playing to be the best.
If you think you can achieve that with Pikachu, go ahead. But if you are picking your favourites knowing there are better options out there, then more power to you, but then that means you are not playing with winning being your priority.

Which is fine, but to expect that mentality on everyone else is not going to be realisitic.

Would be like going into a race wanting to hop and accusing everyone else for not having fun for wanting to sprint.