They can drop weapons. It is just rare. Mostly high level tanks and harvesters. That is how you get 5c attachments. And how you get the 5c weapons that are not placed.
And if you’re really lucky (and have two points in the mechanic(?) skill…) higher level enemies drop extra attachments like vision modules to get nightvision, infrared or x-ray vision. They come as single modules or in any combination of two features and veeeeery rare as full vision module with all three features.
Didn’t get any more FV modules for ages though…
Same applies to seeker horns, nearly always empty…