The problem with mobile games is that they have to be very fast, and GSGs cannot be fast and still retain full features. Very few people will ever invest two hours looking at a phone screen. And if they somehow actually do that, they'll probably own a laptop or such, and play the full game on that instead.
Two things to consider -
A - Mobile games almost always devolve into crappy ad-ridden "freemiums", on top of being already very simplified, in order to attract the zombie horde that usually forms their playerbase. That horde doesn't touch anything other than casual games, but that's where most of the money is. Strategy gaming in general is out of that equation.
B - Even if a fully functional proper GSG gets released on mobile, it is always under the threat of being buried. I don't know about Apple, but Google Play Store is notorious for its famously bad quality control. One has to dig through mountains of trash heaps to find a good game or app. Unless the game has a strong marketing and somehow manages to get itself featured on the app list, it gets inevitably buried under an avalanche of crappy games for everyone except the niche list of people who already know about it from elsewhere.
GSG is already a side market. It would be even less profitable for a dev to put that much effort into creating a fully featured game, unless it is a labour of love like that free OpenTTD mobile port...and that is one of a kind thing. Most people who would be dedicated enough to find and buy such a thing, would likely also have a computer where they may or may not have already tried the main version of the game, because they aren't casual players.
And lastly, PDS are struggling to even keep their PC mainstream GSGs in shape these days (examples include Imperator, or EU4 in its current state, Stellaris a few months ago). I respectfully express a strong doubt that they are ready to step into mobile GSG market, create a separate large team dedicated to recreating games like EU, HoI and CK on mobile devices.
Yes, if they did make such a game there would be many people who would definitely buy them...but I doubt they need to make this, or they ever will. Even if they do, it won't be a full feature game people want.