That is because they ARE.
Once upon a time (2011) there was a long article called "Who killed video games? (a ghost story)". It has now been blown away by mists of time, with only stale links referencing the detailed, cruel and flat-out cynical - but oh so true - treatise into how mobile games are essentially built on top of mathematically optimised engagement wheels making use of the anticipation/disappointment cycle. I wish I could find a working link...
In only slightly unrelated tangent: a university mate was an aspiring game designer. I remember how he once, off-handedly, mentioned how he experiments with mechanics and rules until he finds a set that works. We actually got to playtest some of his work-in-progress experiments, and while I never got to try anything really good, others in the group did.
But right, back to the topic: once the abstract rules of the game itself were ready and sufficiently balanced, he picked a theme and genre most likely to expose the game to the biggest possible (welcoming) segment. It's easy to extrapolate that once a working game design has been found, the creators would fit the same engine with various other skins and themes to target as many lucrative markets as possible.
One can find the same approach everywhere, really. Cookie-cutter reality TV show formats; talent shows; sequels of upon sequels, be that films or TV show seasons; ...
A really cynical view of all this could be that mobile gaming is now a properly mature market. Innovation is unreliable and expensive, so why bother?
Haven't read it yet and not too sure how to use the Internet Archive but this seems to be it's first capture of the story.
You'd think so, wouldn't you?
The final nail in the coffin, so I am not even willing to look any more came with Dungeon Keeper. Dungeon Keeper turned out to be astonishingly cynical crap from an incredibly strong heritage. Micro-transaction and freemium abuse destroying everything they touch isn't solely on mobile of course, yet it seems far harder to avoid there.
My games money now goes to independents on PC only - the mistakes and dull failures are more than compensated for by the occasional Factorio or Pillars of Eternity.
On mobile? I won't even look or try any more - they thoroughly poisoned the well treating everyone as there to be milked.
The version of Settlers I played on the iPad a few years ago is absolutely nothing like that. It was almost a straight port, and the touch screen actually made for a great experience.
X4 games, on the other hand, are pre-destined for tablet.
However gaming in mobile doesn't feel good. The control feels inferior compared to keyboard + mouse, or joysticks.
[0] Pokémon Go doesn't count.
Then imo people don't want to spend 50$ because switch > mobile for gaming.
The games that I avoid like the plague are the in-app purchase games, especially the ones that are just about buying in-game currency. These games are never good.
The sad thing is that the mobile platform should, in theory, be ideal for strategic games and builder games like Anno, Settlers, etc. All implementations of this genre however rely on, after a few hours of playing, increasing build-times of buildings to multiple hours, or days, but allowing you to finish the building instantly with in-game currency. Which is, of course, a crap gaming experience and I am sure the creators know this. It does seem to be a good way to make lots of money, seeing as all games do this now.