The alternative is always to offer your content from your own website, and take 100%.

The game developers who were around before Steam, know how much that costs (promotion wise), and know they get a lot for that 30% cut. The same is true for those other platforms.

You seem to be a person that expects all internet platforms offer their stuff for free.

And another thing: all of those platforms mentioned have competitors, so calling them monopolies is quite strange. It's not because they are market leaders that there is no alternative (which monopolies don't offer)