I think it's usually because it provides a very shallow conversation around design, and because the feedback is rarely constructive (although very very supportive) the perception is that the work all looks the same.
This true. Dribbble is generally not a great place to seek critical feedback. The best feedback scenario seems to be posting iterations in the form of a Gif slideshow or rebounding your previous shot — even then the feedback is very direct in the form of "pick A" or "i like B"
I think it's usually because it provides a very shallow conversation around design, and because the feedback is rarely constructive (although very very supportive) the perception is that the work all looks the same.