The Outer Worlds sells 5 million copies

I wouldn't say "more worthwhile". But comparing them (in my personal opinion): Outer Worlds trades variety and scale for a more narratively dense world.

Biggest thing is you get significantly more choice in questlines. Bethesda's approach in Starfield is very railroad-y, almost all the big questlines end up picking between two distinct options while leaving you thinking "you know we could just do a third one, or both depending on the circumstances". They also, outside of maybe one or two circumstances, have zero opportunity for creative player intervention. If it's not explicitly mentioned as a quest objective, it's not an option. e.g. No, you can't use the EM gun on this guy to bring him in and face justice, the objective is to kill him, so you will kill him and his guards too. No, you can't go and talk to your superiors for backup before confronting somebody over a major crime. Stuff like that.

Outer Worlds is like Fallout New Vegas in that the world responds to your actions as well as dialogue choices. Every NPC is killable, and they've written a number of scenarios (some of them absolutely gut wrenching) for killing certain people at certain points. Big quests tend to present two options which both have dire consequences, but by doing other quests, talking to other characters, you can uinlock additional options or improve how things will turn out. e.g. You can uncover an internal power struggle in a faction and help choose its leader, which changes how a peace talk can turn out with another faction.

Outer Worlds also gives you more tangible consequences for your actions, like changing the feel of an early town if you deprive it of power. The epilogue is significantly more detailed than the one Starfield gives you, covering a lot of minor quests and each major character you've interacted with.

None of that is to say though, that Starfield does not have a rich and interesting world with cool characters. I've loved my time with both games and I think SF has more fun combat gameplay, obviously both are similar gun-based RPG games where you mag dump bullet sponge enemies, but hey ho. SF also let me build and fly a ship, go where I want with it and take pretty pictures, which has been a lot of fun. Starfield may have less quest choice, but it offers more variety in what those stories cover, compared to OW's more narrow focus.

I will also say that SF made a pretty bold narrative decision in its main story that I was not expecting from a Bethesda game. Even though I have a love/hate relationship with how it developed after that, and think the moment itself could have been handled better, I still respect it. OW also really hams up the evil corpo humour in ways some people might find annoying and difficult to take seriously.

A measure of worth between the two games really comes down to what you're looking for in a space-themed RPG. Personally, I think they complement each other very well as distinct experiences.