You can but this is actually a real issue. Full-face masks significantly increase the risk of elevated levels of CO2 inside the mask, compared to a "regular" snorkel that can't really accumulate CO2 (although overbreathing it could lead to elevated CO2 levels in your blood). Elevated CO2 can contribute to dizziness and disorientation, among other things. Inexperienced snorkelers might gravitate to the convenience of a full-face mask but inexperienced snorkelers are the people you'd least want experiencing dizziness and disorientation in the water. And even if they're cognizant enough to pull the mask off, now you have an inexperienced, dizzy or disoriented swimmer in the water without a mask. It's interesting that this issue doesn't really get the attention it probably deserves.