But i just have to ask, since i ask everytime someone bring this topic up ...
How is weapon on back less believable than several tons weight lizard being able to fly ... person being able to defile gravity by his pure power of mind ... or fire that burns without burning anything?

C'mon mate, you can justify anything from this point of view.
II mean, if we have magic how is this:
so improbable?
This is a fantasy world where a lot of things are possible (such as a giant lizard flying) but other things are just meant to be mundane, such as a pointy stick you use to poke your enemies.
It's a stick, it is not meant to be able to stay glued to your back, nor it is practical in any way.
There are *magical weapons* that can fluctuate or magically anchor themselves to you, but this is not the case for ordinary objects.
And it's not like it's a technology problem either, since in DOS weapons were sheathed on the hips of the characters.