one reason OEM gets away with sealant only is because a lot of the assemblies are flat, there's no contoured shape or complex interface with secondary pieces-- think BB oil pan vs. small block.
a second reason is that machining tolerances, fit up and engineering are much better than they were back in the mesozoic era when our junk was made. the parts are all new, and clean and fresh. nothings all beat up and warped out.
also, the technology of sealant has come a long, long way since back when rusty was strokin' it to the sears catalog and all you had was indian head and grease to hold everything together. today there's a broad spectrum of sealants that do a great job across a wide variety of jobs and specialty stuff that works amazing for unique situations.
the biggest issue is all the damn fools out there thinking they're betty crocker just icing a cake with that mess. "the bigger the glob the better the job" is not a mantra to operate by.