Not a lot of people do 300 in a day, but mostly, I reckon, because not a lot of people try to do it. It's not that tough to manage, even time-trialing, over tough terrain, and in lousy weather. Done it many times, and there are a whole lot of folks faster than me out there.
As you get much north of 300, though, things get exponentially more difficult. I agree with Homey: cracking 400 in a day is rather elite, and being about to do that in a time-trial is exceptional and takes a lot of training, resolve, and usually involves an extraordinary amount of suffering.