[zanny]

Unless you use Antergos you need to at least manually build yaourt and its AUR dependencies once.

also, you can imitate yaourt with literally wget <pkgbuild> makepkg pacman -U <pkg>. There isn't a lot of secret sauce to take out of your life by automating those steps after you do it once or twice.

[semi-extrinsic]

One word: dependencies. Of AUR packages on other AUR packages, in an unbalanced tree three-four levels deep. That's why most people use AUR helpers.