It's because while it's true on the company side, this comment is making it seem as if the company is doing this against their users' will, while in reality, most users genuinely want a unified experience across platforms for their apps, as seen by some of the comments here.

It doesn't seem to mention it being done against anyone's will. It's definitely true that a company can save money by only needing to maintain one set of help instruction screenshots, for example.

Right, but you continually conflate "unified experience" with "using custom UI controls".

I never said they must be custom, merely that custom makes it easier to implement.