Actually, building a CMS is pretty difficult and I'd say you want to only use CMSs written by about the top 10%. Not quite rocket science but they're actually very, very difficult. There's a lot of tradeoffs to be carefully made, increasing exponentially as the framework tries to do more (with a lowish exponential factor, but exponential nonetheless) and a lot of ways to make simple things difficult and difficult things impossible. There is no pain like trying to use a CMS written by a bottom-25%er.

Using an already-good CMS to build your site is trivial and you don't need a rocket scientist for that. Depending on your needs you may not even need to be a programmer per se.