If you read it, it was much more than first cousin marriages. There were brothers marrying sisters, Uncles marrying their nieces, first cousins, second cousins....if there was a type of incest, you can bet your ass a 'Royal Highness' has practiced it at some point.
What most CK3 players do is marry their kids
I hope you mean to say that they marry their kids to each other, though that's only relatively (get it?) better than marrying your own kids. Not a sustainable long term sexual practice at any rate.

so they don’t lose shit-tons of land because of the stupid partition system they are using.
Not losing out on land and wealth is precisely why Consanguinity was practiced among 'royalty'. You are basically mad at your historical game for being historically accurate.