I think what gets in the way of making more frequent films is the studio's almost unrealistic expectation to bring in huge box office results with them, or reclaim the glory of the first films. Ridley seemed pretty keen on making heaps more Alien movies, but at the end of the day it seems like it's governed by results and fan reaction, which is sort of a shame. They really should just settle on making decent films with not as much pressure on bringing in big bucks. Personally, I think Prometheus and Covenant did well and most people liked them, but it's still not enough.