I'd say it's a push on plausibility, but I believe the idea in the comics is that Cap's shield is one-of-a-kind and even more than that an alloy that's not been able to be reproduced.
Yep. In the comics it was an adamantium / vibranium alloy; but the word adamantium seems to be tied up in the X- men license at Fox, so the movie leaves that part out.
I wish they had left a little more of the comic idea for his shield in the movies. His shield was meant to be the first piece on a new indestructible tank class; but the only time the alloy worked was when they made the dome shaped piece for the hatch. The shield was seen as a useless piece of junk from a failed experiment until Cap happened across it.