Moving parts have to have clearance in order to run and not seize. These same moving parts need ways to regulate the flow of oil around them while maintaining the critical.clearances in order to run properly.
for instance, valve guides need about a thou and a half to 2 thou clearance (.0015-.002) between the itself and the moving valve. In order to not gall and seize, oil is needed to form a cushion between the 2 parts. Contrary to popular belief, a valve seal does NOT prevent oil from entering the combustion chamber.... it METERS oil into the valve guide to lubricate the parts. So a certain amlint of oil is allowed in the chamber... but too mich would cause severe combustion issues.
So you fan see that maintaining that tolerance on the moving parts and metwring rhe oil to the guide necessitates a seal.
Seals/gaskets are not just to 100% seal or make up for machine tolerances, theyre engineered into the mechanical system as almost a an integral part of the lubrication system. No motor would ever be able to be 100% seal and gasket free, not as long as were using the technology we do right now.