This is out of my league, but my neighbor growing up owned a body shop and I asked the same question once. I don’t remember the exact answer, but as I recall the real difference had something to do with the way the aerosol evaporated as it dried which basically rendered it chemically or molecularly inferior to gun applied paint.
This has been probably 20 years ago. Is it still accurate? Has rattle can paint evolved? Was it ever correct or total bullshit? I can’t say, but it might help you in searching for a good answer.
I will say this, I’ve never seen a car painted with a rattle can that looked good, but maybe I just haven’t been looking hard enough.