Why ride knobbies on a commuter? For me, riding a bike is often practical, good for my health, the environment, etc., but I ultimately ride bikes because I enjoy it.

I'm curious to learn about others' decisions for riding knobby tires on their commuters, aside from practical reasons, which don't interest me.

I tried it for awhile on my CX bike in NYC, and wasn't in love, but I see so many commuters setup with knobbies I can't help but wonder if there is some unexplored land of fun I've yet to experience.

And yes, the 'answer' is: just try it myself again.
One of my bikes, used as a commuter, is kept seriously cheap. I paid as much for the bike as I've been known to spend on a decent tyre. So it has the tyres it came with (well, one of them, I had a slightly smoother spare in the garage for the other). This bike lives outside 24/7 with a high risk of theft, and does 3km/day. Looking cheap is like a third lock. 

That's an extreme example but most people riding around town are riding cheap bikes with whatever cheap tyres some marketing person thought looked good on a bike styled like a mountain bike. Some of these riders are also quite noticeable as they do daft things like running red lights. And it's still much quicker than walking for the same effort.

That accounts for the majority of cases. A few people really like gravel biking and don't like changing tyres. But actually on rubbish roads and with a decent chance of going up/down kerbs even to join official bike paths, a 40+mm tyre isn't so bad, if its smoothish on the centre line