Tried storing my Camaro on grass for 3 weeks and the frame paid for it

Everyone in my car club said a gravel pad behind my shed in Fresno was "good enough" for short term storage, so I parked my 87 Camaro there for about 3 weeks while my garage got rewired. When I moved it back, the underside had surface rust creeping along the subframe and the tires had sunk so deep I needed a neighbor's truck to yank it free. I always heard concrete was overkill and only for long term stuff, but that wet gravel held moisture way worse than my buddy's slab ever did. Cost me $140 in wire wheels and rust converter plus a whole Saturday on my back. Turns out the ground under your car matters more than how long it sits there. Am I wrong for thinking gravel is basically a trap for anyone storing a project car outside? What do you all put under yours?