You're missing a fundamental point here. Cash is FIXED for a given year.
32 teams x $185M = $5.92B. That's all the money there is to spend for all teams.
If you trade, say, $5M in cash, you're essentially giving ONE team more than its fair share of cash, handicapping 31 other teams. Now you can say, HGWHD by fleecing some newb or taking advantage of a tanker, but it's a league killer when you allow that kind of stuff since it unbalances a balanced game.
Put it this way, how would YOU feel if your division foes all had $200M to spend, while you had $185M? (Well, you'd feel like the Orioles, and feel like the system was unfair, and you'd likely bail)