One is a fuzzy and subjective measure of how much software a team can develop in a sprint, the other is a precise measurement in a single dimension. I could sit there and try to calculate how many expertise-adjusted person-hours each team member represents, then estimate our capacity that way, but why fool ourselves with that kind of false precision? Just eyeball it the first sprint and then observe how much we get done. Explicitly tying story points to time just invites abuse of the velocity by managers who don't understand how scrum works.
I frequently hear about this abuse of estimates and even ran in to it myself when I was a more junior engineer. In my opinion, part of maturing as an engineer is learning to call your shots and properly communicate when you aren't sure how accurate you are. It's one thing to say "I promise this will be done tomorrow". It's another thing to say "we're not positive but within the month seems likely"
