I'm not sure I agree. I use one phone, with many apps, so it's helpful if the common parts of all the apps I use behave in the same way.

For example, how do I get to the preferences for this app? Via a hamburger menu on the left, or an ellipsis menu on the right?

Almost all the apps on my phone that have the concept of making a new item (SMS/instant message/email/calendar event) have the button for doing so in exactly the same place on the screen; this is great as it means only have to learn that action once, and it works for all the apps I use. That is optimised user interface design.

Of course designers still need to put in effort to make their apps usable, but making having the common subset of actions work the same across apps is great for users.

It's unfortunate that platforms seem to change their interface guidelines all the time (no just the look and feel), as it seems to be having the opposite of the intended effect. The cost benefits of having a single style for an app across platforms is also a significant factor; I'd rather have an app available on my platform than not.