I can think of at least one reason why a BB or otherwise decent construction would wear out well before it "ought to" and has nothing to do with the frame or install. Load or water exposure (gee... that's two reasons). I suspect that the OP is a commuter given his miles/days a week of use. I wouldn't be surprised if this bike sees a lot of exposure, both when ridden as well as when parked. Commuting can be about the most abusive type of use one can give a bike. Add in lot's of weight and hills and this low life of the BBs isn't too surprising to me. Not saying that I would expect this low mileage or that I, too, wouldn't be frustrated by it. Just that I've seen this kind of thing before with high mileage and all condition riders, especially if there's a lot of weight involved.

I will say that the situation I describe often has other wear issues going on too, like wheel bearings, spokes, brake pads, chains.

I'm not accusing the OP of anything, just offering more experiences that do happen to, thankfully few, others. Andy

I should add that the OP might think about having a service focused and motivated shop (and with a very low cost bike many shops don't go that extra step of thoughtful assessment beyond the flat rate repair) do the next install with a UN-55 BB. I do wonder what the old BBs looked like on removal. Andy