As someone who doesn't know anything about blender or unity I can tell you right now that the whole process of making a custom avatar is so fucking user-unfriendly in its design that even watching youtube tutorials is too much for most people. 
Yes, yes, I know it's real easy and simple and you can do it with your eyes closed yes, good. Thanks. 

But most people can't do it at all, they don't know any of the terms, they don't know what a component is and they don't know they need one at all let alone which ones to do which things. They may try following the VRC tutorial on making your first avatar, this is a mistake that will put them off for life since the tutorial assumes you already know how to do it and is therefore fairly worthless. 

I've been learning myself, I started with no experience at all and now I have "almost no experience at all" so that's progress right but I can totally see why most people would just give up and use an avatar from a plinth. Remember, for most people anything other than basic use of their devices is beyond them.