I think that alcohol causes way more impairment than pot.
It's not that easy. When you say alcohol causes way more impairment, how are you measuring one against the other? Drinking one bottle of beer vs. smoking one joint? There's just too many variables to make a direct comparison.
Is it really so hard to just use Uber or just have a designated driver? There's really no excuse nowadays.
Ontario is a big place. While it's relatively easy to take a cab/Uber/transit if you live in Toronto, many towns have no Uber, no transit and no cabs. So when your only option is to drive, many will drive after having a few drinks. I'm not condoning it, I'm just telling you what happens.