A bet isnt a bet until there are multiple opposing sides matched together. If there is no bookie managing it, then it’s up to the individual participants to do it. If someone says “5 bucks on the big strong guy” it’s not an actual bet until someone comes along and puts money down opposite it.

When the supporting character comes along and says “100 bucks on the other guy whose win moves the plot along”, it’s like he’s saying " I will maych all the announced bets on the big guy so far, and everyone else’s up to a total of $100".

They could take the time to show that person taking out a wad of 5’s anc physically matching the bets already thrown down, but it’s more expedient to just do the shouting.