Why is it collectors prefer WW2 WW1 weapons/

[Vlim]

It also depends on the collecting area, really.

Many guns went through refurbishing programmes while in use, anyway.

And there are import markings and import markings. Some are just awfully done, others well hidden out of sight. To me, such markings (which also include dealer markings and proof markings) are useful to trace the history of a gun.