The iPhone, iPad, and Mac are different platforms, with different operating systems.
There are enough hardware and operating system similarities that you can run some iPhone and iPad apps directly on an Apple Silicon Mac. But developers must give permission before Apple will allow this.
Some developers don't, because
- They fear that their iPhone and iPad apps won't run very well on a Mac,
- They don't want to make the effort to make their apps tailor their user interfaces depending on which platform (iPhone, iPad, or Mac) is in use,
- Their apps have been in the App Store for a long time, and they just haven't thought to give permission, or
- They already have apps designed specifically for Macs and for iPhones/iPads, and would rather have you run a Macintosh app than run the iPhone/iPad version, which might provide a worse user experience.
Other developers do – and some of those go to extra effort to make their apps adjust the user interface depending on whether the runtime platform is an iPhone, an iPad, or a Mac.