Well yeah, but if the person who is in charge of the modal is the implementer, not the designer, then they may not have the expertise to design a good ui.

And many designers are artists ("designers") not ux engineers, so their main goal is to make something that shows their artistic skill - they're going to be trained in design packages, not designing a good ui.

(I mean, when was the last time you saw a clickable button on a modern user interface? nowadays it's just "guess what this shade of grey means")