How much acceleration can you endure? 3g is typical for a rocket launch, 9g is almost deadly. Accelerating to 6000mps at 3g will require 6000^2/(23g) = 600 kilometers of track, and it will take 200 seconds. That's a huge track, which has to be exactly linear or the payload will destroy it.

Then there is also the fact that the payload will be exiting the catapult at the sea-level, thus facing the full atmosphere at 6000mps. Which is to say the payload will be instantly incinerated unless some kind of magic is invented akin to supercavitation. By contrast rockets do not even reach supersonic 300mps until 10k altitude, thus never encounter this sort of problem. Maybe you could build this 600km track thing in the Himalaya, but that much and that high in the mountains isn't going to be cheap.

You can tell I though a lot about the same thing that you did. :)

I know people have talked about this for a long time. There must be a logistical reason for it.

As soon as we figure out how to build 100+ km tall structures.