I'm not aware of any price-matching clause. None of the key resellers would exist if that was the case. Retail stores would be unable to offer sales.

For Steam and Mobile, you need to create your own playerbase. Merely launching onto the platform will get you nowhere. If you are putting a huge effort into advertising and PR to generate an audience - you must think, why not direct them to a platform that charges me less than 30%? Android and iOS are effectively closed shops, but for PC I think its simply that no decent competitor has ever come along.