Both of these make up the answer. There’s a lot to cover and I don’t know very much about the particulars, but to reinforce what freakscene said, every single way Hollywood movies are made today is different: the way they’re lit, shot, staged, colored, edited, and eventually exhibited is handled by new workers with new know-how using new technology and new opinions about how to use that technology following new industry rules developed around the new technology. Even when it’s Scott in charge of Prometheus and Covenant, he’s working with a different crew and they with different tools. Scenes of people standing around and talking are paced differently (nonlinear digital editing and digital cameras inform the shooting schedule, the coverage a director can get; coverage informs editing, shot duration), framed differently (again informed by essentially limitless digital storage), and are nevertheless filled with CGI—not the only differing elements, just the most visible and the easiest to talk about; the crew shoots similar sets from Alien and Prometheus differently and prioritize different visual elements. One small difference: camera moves today are aided by computer stabilization, while you can see the crane wobbling in the original—one potential demarcation between lifelike and video game-like. I’m just speculating, I have no idea what it was like on the set of any of these movies, but all these changes in circumstance could be at play and serve to explain why new movies look the way they do, and even why Prometheus (pre-COVID) looks different from Romulus (post-COVID), besides of course being made by completely different people. (Incidentally: it’s too bad almost all the uploads of these clips to Youtube are weirdly upscaled or compressed, makes it hard to compare.)
Moreover, “special effects” is itself a vague term which encompasses a variety of artforms like makeup, animatronics, backgrounds, lights, composite photography, computer graphics, and a million other things. That Romulus involves practical effects is true, but in the same way it’s true that bottles “made with recycled plastic” may be made from 1% recycled and 99% new plastic, for example. I haven’t seen Romulus but the trailers suggest a reliance on practical effects, at least compared to its contemporaries—something which does help it stand out—but many effects shots, like the one where a character shines a light through her torso to show her ribcage being smacked by an alien, are computer-reliant; I would bet a significant number of Romulus’s “money shots” are entirely dependent on computer imagery. But reducing the issue to computers vs. physical material doesn’t tell the whole story, it’s just easier to discuss than halogen vs. LED light bulbs.
For comparison, every single thing you see in the original is accomplished with physical matter, light, and chemicals. Marketing for Romulus may play up its use of practical effects, and whatever those effects are may serve the movie better than the complete reliance on (underfunded, rushed) computer stuff in other Hollywood movies today, but it doesn’t really work to compare it to a monster movie from the seventies (or eighties or nineties) because of how different the palettes are that the filmmakers are working with.
All that being said, with all its resources the studio (Disney) would certainly be capable of recreating the exact look/texture/etc of the original Alien, if they felt like it: they could afford the film, the cameras, the lights, everything. Getting ahold of enough Eastman color 5247 film stock to shoot a movie would be no problem for Disney money, and the laboratory which developed Alien is still around, but it would of course be cost-prohibitive and basically pointless in capitalist terms—it’s much easier to talk up the use of practical effects in marketing and let this kind of metatextual discussion (which is itself based on intentionally incomplete information) sell tickets.
None of that fully answers the main question…
Speculating again but in addition to all of the above I believe it is born of a reliance on post- rather than pre-production work (cheaper)