I'm not an aerospace engineer, so don't read too much into this, but: In James Fallows' China Airborne, there's a paragraph or so where he talks about how building a modern airliner is harder than building a rocket ship. Roughly it's because 1) rocket ships are allowed to fail on occasion and 2) planes are subject to constant use and are not subject to the kind of preflight inspections rockets are.

In the case of SpaceX, consider their last launch failure. The general consensus seemed to be "well, rockets are really hard. And it was the strut manufacturer's fault to some extent. They'll do better next time, and they're still changing the world. Anyway, SpaceX rockets are 4x cheaper than any competitor so even with the occasional launch failure it's still the best option!"

Can you imagine the reaction today if e.g. a Boeing plane blew up on launch? In fact, recall all the criticism Boeing received after issues with the 787's batteries...

But again, I certifiably don't really know. Someone could write about energy requirements and wet/dry mass and how thin rocket skins are and aerodynamic forces and easily convince me rockets are harder.