it's not really impressive... Domain probes usually scrounge up like what would be tiny tanks...
The survey ships on the other hand do assemble relatively big fleets, but rarely anything that high. You also have to remember that most of these are in terrible condition, and judging by the fact that derelicts roam around with the ability to reduce the supply cost based on the amount of d-mods on their ships (which they have a lot), it's called Derelict Operations after all, and can be found in Derelict bounties. Then it's cheap to maintain a fleet for them. Though these fleets are usually kinda petty with the exception of Guardians.
I also think that the reason why you get so little metal isn't necessarily because of the ship being old, worn down or small... But gameplay. Dumping 1000 pieces of metal onto the player inventory while kinda funny, would also be really annoying. Cause it would encourage the player to pick it up and trade it, even though it is ridiculously inefficient. So it would be sort of a noob trap.
Also, if derelict stations gave more supply, machinery and other goods, then derelicts would have to be reworked to be more scary, which would probably take a lot of work...
Currently they are kinda inefficient, mostly serving as canon fodder to larger Guardians which are designed without any balance in mind, having ridiculously low DP cost, endless missiles and a quite decent mobility. This is also why you can't acquire them. Their stats are either a placeholder or purely designed for NPC fleets.