Software similarity and market positioning don't really come into consideration once the role of gatekeeper has been established.
Also, it costs the vendor to implement support for installing other software - resources the vendor could have spent on features I value, rather than features I don’t want. If only a government didn’t dictate to the vendor what it should do, stripping the vendor and the user of the power to decide for themselves.
And the resources needed to make an app installer are not nearly as high as you make it out to be because iOS already has the mechanism to install signed .ipas. All that's needed in theory is a check to disable signing (which they already have implemented in MacOS) and to add a few pages to the Settings app, which surely shouldn't be an issue for a tech company of Apple's size. And if you argue that it might break some spaghetti code then maybe that should be fixed anyways and it's doing them a favour.