I think its because they already started going before they got up to go to the toilet.  So as they sit there, pre-shitting themselves, it is smashed and spread out by their massive ass cheeks.  So when they sit down, they really already have shit all the way up their crack and all around not even close to their asshole.  Then when they wipe (if they even can at all) they only go for as close to their hole as they can get.  They leave the flattened ahit smear on their crack to spread, dry, and crack off later.  And of course they don't wipe the seat after because they can't turn around or bend down to do it in our tiny tiny stalls. Most of them probably don't even look to see if they made a mess though. cause fuck everyone else , right?  Cleaning is someone else's job.  I imagine even if they did wipe the seat after, they probably wouldn't bother to use a cleaning wipe.  Maybe just scrape the poop off.  Imagine how many seats your butt has been on that they flaked the poop off without dissenfect... Maybe its best they leave it there. At least we know not to touch it.