In my misspent youth, I was able to a few times. Here's the thing. It felt like absolutely nothing. I just said to myself, wow, I have my dick in my mouth but that was it. From an evolutionary standpoint, we can see why this doesn't work. If it did, we'd spend all our reproductive years curled up into self-pleasuring balls with our butts at 12 o'clock. Apart from not reproducing, we would become vulnerable to other predators.