This is not about productivity, this is just about quality of content. You can still make a game of the same standard as way back, by one person. It's just that games of today are objectively so much better and more complex and intricate.
Arguably, games like Fallout 4 have a huge world but it's not complex at all. It looks more realistic, but the storyline is simpler and far less memorable than Fallout 2.
I'd also argue the most complex games are something like Dwarf Fortress or Ultima Ratio Regum. Both of which were made by 1-2 people.
There are games like Rimworld which try to imitate DF's charm, but go further with a team, professionalism, and better funding. But while it's more polished and "balanced", it doesn't come to the same level of craftsmanship. You also don't get the same goofiness as a necromancer who slices off your finger, animates it, then pokes you in the eye with your undead finger.
It's full of crazy details, like falling down the stairs, cracking a rib, and dying when something pushes the rib through a heart. Or that time when someone found out mermaid bones were valuable and set up structures to trap, breed and "harvest" mermaids.
