No, 99% of the reason they use it is that they were first to market, made it mandatory for their first party games that were extremely popular at the time (and even today) and became defacto mandatory for many third party games as it made it simpler to control piracy to just sell through them or include a key in the physical copy and force people to install Steam. The majority of Steam users are casuals that couldn't care less about their forums, cards, social profiles and so on. It's the same thing in everything, there's enthusiasts that think everyone is as crazy as they are about their hobby, the majority are just casual users that will never know/use half of the possibilities available to them because they don't care.

Lol. You think 99% of people give a shit about forums or Linux support?