They take a 30% cut of every game and make billions. They are not a free service like, say, Discord is. They should not be showing popup ads.
People are up in arms about Windows 10 showing ads in the Start menu, but somehow Valve showing them in a popup which you have to actually close is okay.
Steam is not a paid service.
I do think they should lower their 30% cut, though that is the industry standard.