You're argument rely on the assumption that entering the smartphone market now is as easy as it was when Google and Apple did it, which is ludicrous.
Edit: even if it was as "easy". It's not like there are dozens of companies the size of Google and Apple that could pull it off.
Doing nothing is unfortunately a complete opposite of what they are doing.
Just because a market is hard or expensive to get into doesn't mean it's not a market worth entering. The real crux is Epic doesn't actually want an open phone platform, they just want to be software rent-slurping middlemen for an arbitrary 'cut' of a pie.