Concept: Herding

Herding is when a game pressures players to play together, to the disadvantage or elimination of solo play.

Some games outright require multiplayer. Shattered Horizon was an MP-only team shooter, didn't even have bots. (SP was later added in an expansion.)

Some games have modes that require multiplayer. You can solo a lot of content in World of Warcraft, but you need to join a group to raid.

Some games are more challenging, less rewarding, or less designed around single-player. To a degree additional challenge can be its own reward, but something balanced for multiplayer isn't always enjoyable or even possible by yourself. In Warframe, you'll get loot faster together, as well as having new combinations of powers available.

I imagine that developers think promoting or requiring a social aspect of the game will increase participation and longevity, but from experience I can say it's more likely players will skip the game because they can't get people to play with them, or quit when their group stops convening. It's always easier to schedule for one person than four.

Pickup groups have their own problems. They can't reliably be assembled except in the most populous games. New players are often a liability due to mechanical mismatch (like level imbalance), clashing degrees of player skill, selfishness, and simple unfamiliarity.

For me it's a plus when a game doesn't push me to group up.