I guess there's at least two arguments for that.
1: It was a simple, bare bones mining ship.
It had the basics to do what it needed to do with no fancy fluff.
2: It was made in 1979 and that was the director's future vision at that time, given the day's tech and more likely, the budget he had to work with.
We couldn't have spaceship tech looking like that in a film made these days. It has to reflect the times.
The ship in Prometheus was basically a trillionaire's no expense spared luxury yacht to ferry him on an all or nothing last voyage to meet his maker.
The ship in Covenant was a high tech colony seeding vessel, so it's gonna be a bit more advanced than your average high mileage mining ship.
Saying all that, you're just gonna have to go with it and accept that times have changed
If Alien was made today, then the ship would obviously look more modern and flashy, but it ought to still look a lot more basic and grimy than the Prometheus or the Covenant given it's purpose.