There are more single-player games than multi-player, but overall more hours have been spent (or are currently being spent) on multi-player games. For example, someone might play BioShock Infinite once and complete the story in under 15 hours and then never touch it again, but easily rack up hundreds of hours in Apex Legends.
As you've said though, there's a sampling bias. Going by the "current players online" metric might diminish how many players are playing single-player games without being online.