Not sure about that: all swimming snorkels I've seen in the wild have a mouthpiece; looking at google pix, the ratio seems to be 50/50 at best.
I’m not so sure that the mouthpiece by itself is the culprit. Rather the materials that the mouthpiece is made of. Decent quality snorkels with silicone mouthpieces aren’t usually the source of discomfort. The cheap vinyl snorkel sets on the other hand…
But, I don’t think that’s the problem these snorkel masks are trying to solve. Looking through the New Diver section, you can find examples of people that have trouble, initially, with breathing through their mouth in the water. That’s the target customer. It’s an equipment solution attempting to overcome a skills problem.
When I first saw these hit the market, I just asked, “Why?” I just couldn’t see the reason for it. Breathing through a snorkel or regulator just wasn’t really a problem for me. But, I grew up in and around the water. I outgrew fins as fast as I outgrew shoes. Then, I realized I’m not the target with these. The target is those who are less comfortable in the water, and may have trouble with switching the valve from lungs to nose to lungs to mouth automatically.
These do solve that problem, but that may be a big part of the problem. This opens up the snorkeling world to those that it would not have been open to. These are probably not people that are quite as comfortable in the water. So, it seems reasonable that these may have a higher rate of problems. Nothing to do with the mask, but rather what is behind the mask.