Velosock is a brand I've seen. It's more meant for sheltering bikes on a car carrier, so they are more conformal to deal with wind. They are really expensive, and the normal versions aren't even water resistant. The mid-level version is water resistant (but not water proof). The most expensive version is allegedly waterproof, but it's hard to make a stretchy fabric waterproof, and it needs to be stretchy to fit a variety of bikes and be able to withstand wind loads.

I think it wouldnt be too hard to DIY a cover that would be good for a parked bike. Something like a silnylon fabric is very lightweight/packable and plenty waterproof. For example, I have a poncho I made from 1.2 osy (42 gsm) silnylon that I think would work well for a bike tarp. You could just do a single seam to make a square tarp and put some grommets on the corners for ~$50. Should weigh ~300 g.

Part of the problem, though, is that rain means 100% humidity, and even if you've covered the bike, there will likely be condensation on the metal.