Keep in mind two things: In a bunch sprint at the end of a race, the racers are already at about 26-30 mph before they even start their sprint. Everybody has the same gearing (presumable 53/11), so he with the strongest legs, spinning the fastest, for the longest time, without tiring, should take the win.

The HPV record holder probably had a HUGE gear that got him the top speed. When he got to 50 mph, he still had gears left. You can't just start from a stop in the gear that allows you to cruise at 50, you have to cycle through the gears, in effect 'spinning out' of the gear you're in before you shift.

Going from a standstill isn't what anybody really does to get to top speed right away, and the reason you're done at 28 mph is that you're tired.