Why should they? Ok, if you know the modeling tools and know exactly what you want, modeling it yourself probably is the only option. But if there is a readymade prop that fits in, why not use it?
Not everyone has every talent needed.

If you look at the pro studios they have modelers, riggers, animators, shader programmers - everyone does what he can do best. So why should an amateur master everything, from modeling over lighting, from a natural pose to perfect shading and texturing?

Thinking further, even people who do so still rely on work done by others. Every renderer, every modeler, every shader was created by unmentioned people - but without these, 3d-art is simply not possible.

It's great that we have the tools now, it's great that a lot of people can use them. Art is not skill - and that you need much less skill now to produce art is a good thing. Skill still makes a difference, but if I see an idea, an emotion ín a picture, it's art for me. And I see that often.

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.