"You shouldn't joke about things like that."

Basically its the same reason that many other large game company ends up using their own launchers instead of steam. Steam charges something like 30% of what the games make to host it. This is actually a pretty reasonable rate, other publishers usually end up charging significantly more (40-50%), which makes it great for smaller game companies who wouldn't get much recognition otherwise. Once game companies get really big, they end up making more by using their own launcher instead. This is what leads to the "thousand launcher" thing everyone complains endlessly about.