The thing is that no one likes their deal to get vetoed. I've had it happen and I haven't liked it. But, if 10 owners can agree on something, anything really, you have to at least give some credibility to their collective opinion. Unless it's some vendetta veto(and I've had that happen), the trade probably was one that significantly shifted the balance of the world. And when a world becomes unbalanced, you've got a problem.
I'll use Hamilton as an example. I'm in my 4th season there. I joined in S6. It's a pretty good world. No teams are winning/losing 120. But there are some teams that are always bad or always good. I don't know what happened in the first 5 seasons but something did. Now, in what is an otherwise good world, we have 6-8 openings every season. Where do those 6-8 openings come from? The bottom teams. I think we had 6 openings last season. 5 of them came from the bottom 12. It took about a month to fill and, due to impatience, some newer owners were given the teams. Can a new owner turn around a bad team? Sure. Can he do it if he's making bad trades? Nope. What if the trade are the best he can get but they're still bad? Nope. So, if you say "Let them learn from their mistakes!!", they probably will. But they'll take their knowledge to another world and leave behind a worse team than the bad one they took in the beginning. Then the world will sit for a month or more waiting for someone to take that turd of a team.