90% of developers are releasing shitty or niche games with no sense on how to market or monetize to the audience. It's the #1 problem of the indie game developer right now- they're building games that they themselves want to play, instead of the games that actually work for the market.
It's the difference between a professional and a hobbyist. If you want to make money, stop being a hobbyist and turn into a professional.
For professionals, none of these engines are cheap by any means. Unity is $4500 per developer once you add Pro, iOS, and Android. Unreal / Cryengine are potentially going to cost you a whole lot more than that given the 5% gross revenue share.
For hobbyists, yes, the price is low and probably not sustainable, but for professionals I think it's priced perfectly.