Why on earth would EA decided to put their games on Steam and lose 30% of their revenue when they have their own successful platform?
Business 101...
70% of something is better than 100% of nothing

Also, you do understand that "figure" you pulled from your butt is negotiable on an ad-hoc basis? Not a "locked in" figure.
Point being, Valve will NOT ask a major publisher for 30%.
Reason being, 1 million titles selling at 15% profit is better than NO titles selling at 0% profit.