Why not use game engines for cross platform development?

I suppose this might not be a Stackoverflow.com question, and if not I would like to know which of the network sites allows this kind of question as opinions here are actually exactly the kind of thing we need to have this question answered. But here goes anyway..

Back in the day, the argument I often heard against developing B2C web apps in Flash was that SEO was important and that Flash was no good for promoting keywords etc. Then the world of web apps was upended by the rise of Mobile, and suddenly everyone is trying to deliver services by Mobile app, where SEO plays no role whatsoever.

Now we have a situation where getting services delivered is ridiculously time consuming and expensive as we have to develop multiplatform front ends and test across a fragmented spectrum of devices. It's 2016 and only now technologies like Xamarin are starting to get mainstream traction, debates still rage about hybrid apps, and no one person can agree if a single codebase approach for web+mobile is viable or not.

To address this kind of problem we have Crosswalk for hybrid, which bloats out the app to 20MB (so what?) and Xamarin for multiplatform native, which is great if you don't want to develop a web app on the same codebase.

So now we don't have to care too much about search engines hitting our apps, and we don't have to care about large downloads as in any case the alternative is trying to test against a gazillion types of device with different Android versions, different feature support etc.....

This just makes me wonder if the best way to do anything at all for front ends is not just on top of a game engine, like Unity or Phaser or whatever, deliver a wicked UI and treat these platforms as the new 'OS.' I really wonder if the only real reason this isn't happening is just cultural inertia and lack of skills, or if anyone actually has a good reason why they aren't doing this? :-)

Opinions welcome!