Getting into orbit requires a lot of 'delta-V'. Part of this is gravity (needing to move up the gravity well) but more important are speed (you need to go really fast to stay in orbit) and aerodynamic drag when moving through the atmosphere.

There have been no real improvements to the exhaust velocity of rocket engines (at least none that work to get into orbit). As such, to get more delta-V we need to do that with the dry-full mass ratio.

This means we really care about the dry mass of our rockets. This in turn pushes engineering to the limits. The sweet-spot between weight-savings and reliability is skewed more towards the weight-savings side.

Beyond the rocket equation, there is the issue of little iteration being possible. Launching rockets is expensive, so it is not done often. Moreover, failing to launch a rocket is even more expensive. This makes it take a long time to figure out what works by real life testing. Combine the weight skimming with the massive forces and complexity of a rocket, and you get some problems that can essentially be found only by real life tests.

As real life tests are few and far between, it just takes longer to learn the lessons needed. This is something space-X has gotten quite good at, by always iterating on their design and doing tests. This is helped by the relative cheapness of launches for them. Cheap launches means you can try more per launch.

[1] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation