Living close to a university where I got many opportunities to work with dead people (morgue) when I was younger (my friends mom was a professor), seeing weird and disgusting things have always been a part of my life. I don't gain pleasure from watching or looking at "weird" things, its like you said, something I've become desensitized to. I think some things on that site are cool, but I don't like everything that is fucked up. I can just handle a lot more than the average person.
People have always enjoyed, participated, and attended (not saying I do) weird events in history; torture, decapitations, gladiator like events, crucifixions, boxing, ufc, etc. so I'm not too surprised people enjoy some weird shit. I haven't met many people who enjoy this stuff, but then again haven't talked about it with many people either lol.