You're being downvoted but you're absolutely correct. Even today, in the 21st century, the US Congress mandates Solid Rocket Boosters in all NASA heavy lifters

You're being downvoted but you're absolutely correct. Even today, in the 21st century, the US Congress mandates Solid Rocket Boosters in all NASA heavy lifters, as a Congressional mandate with strong oversight on design to ensure compliance. The reason is purely political. There is no engineering or scientific benefit to SRB's, not a single one. But there are benefits to propping up SRB companies with NASA purchase mandates that we source ICBM stuff from.