Because most people don’t have a clue or an interest in Classical Mechanics. Most people think rockets go straight up and turn left to get into an orbit. And they have no clue why they go into orbit either but they do know going into orbit is what rockets ultimately do.
Escape velocity refers specifically to unpowered objects. It has no relevance to powered machinery.
If a machine such as the Dawn probe wanted to use an ion thruster for months at a time, it could power it’s way out of the solar system using continuous thrust. But this is not the same concept as using a traditional chemical thruster to give a payload enough thrust to reach a velocity at which it could coast right out of the solar system without any further thrust.