While RTV silicone is great stuff, your question should be more why use gaskets rather than an RTV silicone sealer. Reason, in an ideal world mirror polished surfaces would seal by mating them together strongly AND be able to take them apart for service. There in lies the rub...gaskets make up for uneven mating surfaces and compensate for temperature, vibration, fastener stretch/tension and such, the real world subjects to mated sealed surfaces YET allows ease of maintenance and reinstallation.
One of the problems that would happen if you used sealer would be gasket not letting loose when trying to take mating surfaces apart, the other problem is cleaning and prep before remating the disassembled parts after service, not easy if gasket or sealer if truly "stuck" to one or both surfaces or traces are left over to not allow the new contact surfaces to mate cleanly.
Stick with using gaskets and only use sealer or grease as an assembly tool and only on the removable surface, NEVER on the fixed surface lest you make it next to impossible to get off or clean up when servicing.