In some respects I blame Tokyopop. During their phase of The Rising Stars of Manga contest and publishing OEL manga, they took on a lot of young artists who weren't quite ready for it- and artists who were only vaguely manga-inspired in their art. I think it gave readers this sort of feeling that Westerners just can't draw manga. It doesn't help that a lot of readers were quite young and immature back then!
So the few artists who were already professional quality suffered and in many cases moved away from manga, and the promising kids mostly took the lesson that they couldn't draw manga and also gave up. I've seen a lot of promising young manga artists from that time period (what was it, late 2000s?) who now don't draw manga at all. It's really sad.
Personally, one thing I really like about manga is the unity of style. Works are similar enough in art style that it's comforting and familiar to readers, but it still leaves plenty of room for individual art style and innovation. Plus, artists who do have really unique styles stand out a lot! Whereas I get overwhelmed by how every webcomic out there seems to look totally different! Every time it's a huge adjustment.