I’ve got wallet and lighter front right, phone and cigs front left, and keys on a carabiner tucked into back right. Back left is basically always empty.
I've done the same for years, but am thinking of making use of the second SIM slot in my personal phone and using profiles to separate everything. Many phones can also use an eSIM, so that's also an option for multiple numbers.
In the Netherlands, there's no duty to carry ID with you everywhere you go, but if you happen to be asked to show ID by someone with the authority, you have to, so it's sort of mandatory in practice because you never know what'll happen.
Oh, I thought you were asking why people were carrying wallets. My ID is in there, is my reason. I can't carry my credentials on my phone, so a slim wallet is my answer.