The minimum size slf bay is 5, isn't it? That's 32 tons of cargo you'd be sacrificing for something that is for all practical purposes nothing more than an additional mining laser. It would have to be waaaaay more than that, or else it would be utterly useless.
if it mined via laser, it would be more than just an additional laser since it would double your yield because it would be another ship mining.
or it could laser mine a nearby roid you prospected while you core mine one, then you can scoop things up after. or maybe it scoops and brings things to you by transferring cargo from the slf dock. or perhaps it allows precision laser mining to get better yields etc if piloted by a human
or maybe it doesn't laser mine and augment mining that way, and instead it is a super sized limpet that doesn't die and can scoop more than one thing at a time.
those ideas don't involve changing the surrounding mechanic and i think would make it worthwhile.
i personally would prefer a miner slf that allows salvage gameplay though. cutting up wreckage and such to create new types of commodities that players can use to repair or even fully fabricate ships and modules without buying from a shipyard ...a whole new mechanic centered around self sufficient crafting. but that's getting into wishful thinking.
why we dont have it already...i really don't think can be attributed to there not being a good place for it in the game. we knew about the new srv for years before it was put in the game and nothing about it needed foot combat for it to do what it does, so why it wasn't added sooner has nothing to do with the purpose for it not existing. so i choose to think we don't have this miner pod yet for some unrelated reason to its balance placement in the game.