In Japan (where Tokyo Marui is based, and who they sell their products to - they have no interest in markets outside of Japan), there is a law that says the guns must shoot under 300 FPS.
My understanding from hear-say is that the guns should not be built to exceed that limit...Or something along those lines. In other words, don't reinforce the guns to shoot over 300 FPS, because they should not be able to shoot over 300.
Maybe there's some law that they have to be plastic too. I don't know. I should not be spreading rumours and speculation on the internet.
But outside of the laws, the plastic slide is sort of necessary. With such a weak blowback from only 300 FPS, a metal slide would be sluggish.