OP's argument is valid. The Raid "Roulette" has the same issue, defaulting to P12 most of the time despite the vast variety of options available and no indication that there isn't anyone queueing for them. Not even P9-11 show up as often.

The claim that the roulettes are attempting to fill someone's party has some merit, as you'll occasionally get a fringe raid with a DPS party leader who's also a first timer, but to say that it always does this is a misconception. When it picks P12, there's a good chance it's putting 8 different rouletters in there. And this logic completely falls apart when you remember where the Alliance Raid "Roulette" sends you, while people wait hours trying to do the Stormblood raids.

Saying "MSQ dungeons are bound to pop up" also doesn't make sense when the Leveling roulette...consists only of those. It doesn't explain why only those few dungeons in particular would continuously show up for him and not all of the others. Many of them have minions/music rolls/cards/glamour too.