Short answer: you still need a complex vehicle to finish the orbit insertion after getting thrown, and now you have the added complexity of designing your (smaller) rocket to a) survive a massive g-force while being thrown and b) fit into the catapult sling. So far the tradeoffs haven't been worth it.

Basically the only things we know of that can survive our spin launch systems is stuff like 'very crushable food supplies' or 'a giant lump of steel'.