Well, you’re missing one detail. The risk 55 for 120 option is a parlay between 2 teams. The 55 for 50 option is only a one team bet. In a way of speaking the first bet is twice as unlikely as the latter, ergo twice the reward.
Also, when you say “they win when you don’t, which statistically, is a good take” isn’t accurate. The point spreads are intended to create a situation where the betting on both sides is equal. Mind you, this isn’t the same thing as predicting the actual difference in the expected final score.
Baseball (and boxing) don’t use spreads; they lay odds. Betting against a spread is a different animal; the vig is not collected on “even money” (against the spread) bets that win.
Because the book tries to balance out the action on both sides of the wager, you could consider it a 5% vig on all action, but in reality it’s a 10% vig on the losing side only.
If you never lose, you will never pay a vig. If you did, you’d only get 45 on a 50 bet. Risk 55 to win 45? Good luck getting any action on that book.