I don't think that they all sound the same, I think that, like with every genre that gains exposure, a lot of bands gain exposure with it, and with that exposure comes more listeners, a lot of whom now want to be in a -insert genre here- band and it's easy to get lost in the sea of em. I'm not saying it's a bad thing either, if anything it's good because there's a lot of music to delve into but the downside is there are a lot of generic bands that you have to wade through to find em sometimes. At one point I'd listened to so many generic thrash bands that I was adamant that I'd heard everything worth hearing (an ignorant point of view I know) so when my friend wanted to play me Overkill (I was fairly late to the party there) I almost refused, cuz I thought they would be just another thrash band, and I'm glad I didn't. I saw the same thing happen with metalcore for example, for every band that I thought had substance, there were 10 who I thought didn't (still remains, all that remains, it dies today, the agony scene etc.) who came out of the woodwork when the record labels smelt money.