Because people think, just because the have an audience of potentially millions, that alone makes their opinion in any way relevant. They mix up reach with relevancy, so to say.

Btw, at the same time you have to accept that everybody has different tastes, and therefore, there certainly will be people who don't like Gal Gadot. Which is perfectly fine, as long as they don't expect that everybody else dislikes her, just because they do. And it goes the other way round as well. ;-)