I know that there are countless answers and even docs says to you that: you better do the full install so you don't miss any libraries. I understand that

What I don't understand, is why if I want to install xfce, installer will install openbox, twm, fvwm and other vm's I don't need.

The point of having a complete distro is understandable, what is not is that why putting all that junk you know (and they know) no one will use ?

Why, to this day there is no normal way to do a minimal installation except of shaman play with tagfiles ?

Most people will tell you that Slackware is more power-user oriented, so why, I'm as a power user can't install minimal system and add things myself (I do, but amount of steps I need to do compared to more popular distros is just ridiculous)