The risky thing about solids is that you cannot abort them. Once they light, there is no turning them off.

Shuttle always started the main engines for a few seconds (wasting fuel) to make sure the engines worked before lighting the solids. But there were many engine-out scenarios after the solids were lit that would result in loss-of-crew.

Congress insisted on the boosters to be made in other districts distant from the launch sites, so they were forced to make them in segments instead of a single continuous tube welded together permanently.