I love building ARs. Its pretty bad actually.
My first AR I bought and I bet 90% of AR owners purchased a built rifle before discovering just how much they love the platform, and then grow into builidng'em.
Cheaper. ya sure it is to build but for a guy that hasnt shot a newer AR in a while, he doesnt know what he wants. He cold build one and probably find himself swabbing parts anyway to custom fit his needs and likes.
Buy a cheap starter AR. A forged lowers is a forged lower, made in one of few factories in the USA, that is your platform to grow on.
Just get a AR with a barrel length you like so you dont need to swap it out. Everything besides the BCG, barrel, and lowers are relatively cheap, and those components, odds are you wont swap them out anyhow.
Rails, triggers, grips, stocks, brakes, pins, charing handles, buffers, springs ..... cheap and a ton to pick from. Have fune man.