That’s typically because the pump design has poor scavenge(suction) on the sump, or is worn the fuck out. Hence why its a damn good reason to pack new oil pumps on new/rebuilt engines with a tacky prelube as that almost always ensures the pump will scavenge and prime. If you’re having a hard time getting a pump to prime on a motor that’s been sitting for an extended time, loosen the filter to reduce the back pressure on the pump until oil flows from the filter housing.
All this BS is why I force prime a new motor with an accusump-style setup I made to ~90psi through a main galley port or off the filter housing via an adapter before I even start to crank it over to build pressure on it’s own. System is primed and to pressure almost instantly upon first fire up.
All nonsense in this case though. fire up, look for leaks, listen for weird noises, watch oil pressure/temp/coolant temp based on gauges in car. Fresh gas fresh oil, go light up tires. I just fired up the lemons car which has been sitting outside for over a year now, still has the same gas in it from ~4+ years ago when it raced. fired right up and ran no problem.