Er, NASA want to do just that, then move to the CEV programme.
But, the ISS needs certain items that can only be launched by Shuttle.
The Hubble needs a servicing mission.
What NASA do is risky, everyone involved is a volunteer, all spaceflight, even after well over 100 STS
missions, is effectively an experimental flight.
That it was once billed as otherwise was a political necessity.
If in the past, we had the attitude NASA face now, we would never have had aviation advances, maybe no aviation at all.