The point being that for a lot of people, it isn't difficult or a stretch to put out that power. So to average 20mph isn't a stretch for them either
Fine, but once you factor in traffic lights, steep hills, tight country lanes etc your mean speed will be much lower than 20mph, so to compensate you have to go a LOT faster when you are riding, which is much harder. If you had a flat or gently undulating course with no junctions, traffic lights or tight bends, then yes a 20mph average would not be a problem.
Your 220W figure is reasonable for 20mph, and I used to be able to average 220W for five hours - that's the average power recorded on powertap, which meant consttant pedalling at that power, even when going downhill or stopping. I never got above about 18.5mph average speed though. I could still probably average 220W for 3 hours but round here that would NOT get you a 20mph average speed.
So if someone says they "average 20 mph" then the real question is:
"how much of that is spent climbing?"
It's not just about the climbing, it's about how steep it all is. I can average 20mph up gentle gradients for ages, and reap the benefits on the other side at 26mph, but on a country lane I might find a really steep hill that has me down to 8mph say and the other side is steep, narrow and windy so I am on the brakes a lot doing 15mph. But both routes could have the same amount of climbing.
You will ride a lot faster than if you do the same route on your own.
I don't find this. Especially not with a power meter. I pick a power I think is going to be a stretch, and I re-evaluate that target as I go, increasing it if I'm finding it easy. If that lets me catch the guy in front then fine, if not then he was just faster. And couple that with the fact that you might still beat him even if you don't catch him, it just doesn't get me going.
MTB racing is different, the short sharp climbs I like involve a flat out sprint, and those can be done harder given more focus on the race. Plus there's the singletrack and descents, where you take risks and push the boundaries of control and technique. Far more fun for me!
But I guess that's why we have different events, because they suit us differently. Froome vs Wiggo in the Tour is the obvious example.