I mean I've casually played some CoD Mobile, it's crappy but I enjoy it well enough. Did some ranked games, and within just ten or so matches the blurb came up saying I "beat" 70% of the players. I'm fairly sure that just means there's a very long tail of players who haven't played as much. I'm sure I could get top 10 or even 1% if I just play more often, moreso than if I get good at the game (because I like to think I'm very middle of the road, and / or that other players are bad because they're 8 years old).
The hint is your words "have to improve". Simply grinding doesn't mean you'll automatically improve, or improve fast enough to ever reach the top. You need something more.
When you are playing games at very high volumes then you also tend to improve rapidly, since even in the 95th-99th percentiles there are usually dumb mistakes that cost you the game.