That works great until you accidentally connect one side of the split-phase service with the other side… Your male-to-male cable becomes a short across 240vac.

Also: if you don’t turn the main breaker off first, you are now back-feeding power into lines the local linemen expect to be dead, potentially electrocuting them… This is why specific receptacles connected with a generator interlock kit (breaker that can only be turned on while the main is off) are used.