Why can't the thunder just move Jeremy Lamb, Reggie Jackson, and Kendrick Perkins for Brandon Jennings and Greg Monroe like I want them to??
Because they like being a good team.
Why would the Thunder be a worse team? They'd gain one of the best passing big men in the league in Monroe, and a guy who does pretty well at defending 5s, who is at least as good a rebounder as Perkins, and a much better scorer by an expodential degree. If Russell Westbrook comes back after the break, Jackson's role diminishes, and Brandon Jennings would eat second units alive.
Both of those players are below average defenders at best - particularly Brandon Jennings. He would eat 2nd units alive but you couldn't really play him big minutes at SG next to Westbrook because he's such a defensive liability. That's one of the most underappreciated aspects of Jackson's game. As far as Greg Monroe - that's an interesting idea but he'd have to become a better defender, it can't all fall on Serge Ibaka. I know people like to overplay the demise of the center, but the western conference still has the likes of Marc Gasol, Tim Duncan, Dwight Howard, Andrew Bogut, Demarcus Cousins, even an emerging Deandre Jordan. I don't think they get away with having an such a subpar defensive 5. They'd be better off bringing in a guy like Omer Asik.
You should look at Monroe's synergy numbers. He does well in post up situations. His biggest failings are as a defensive anchor, and help defender, which are also Perkins biggest failings as a defender.
Jennings isn't a great defender, but he has a lot of the tools to be an adaquate one. But I don't think you'll see a horrible drop off from Jackson's defense to Jennings. Their biggest loss will be Perks lockerroom influence and Lamb's role as a traditional 2, but I think between Thabo having a bigger role, and Monroe's added offense, they can compensate for the loss of Lamb on offense.