Probably because the studios (and some influential individuals in the business) are already “invested” in the real things...and you also have the artiste/nutjob directors with extreme attention to detail, such as when Michael Mann insisted on not only genuine Thompson SMGs for “Public Enemies”, but genuine M1921 examples, that had to be hunted down and refurbished at considerable expense for the project.
True enough I suppose, but most productions don’t require that kind of fidelity to detail. As far as the Thompsons go, a good prop house could’ve reproduced everything in exact detail.