Because they are the best at what they do on the entire earth, playing for a league that is far and away the highest level of competition and highest generator of revenue in the sport. You (and many of the generally pro-business Utahns that inhabit this board and fanbase) seem to have this absurd notion in your head that these are just some thugs picked up off the street and given everything.
I don't like guaranteed contracts either, but that's what happens when you gain leverage for being fantastic - as a whole - at what you do and be an ABSOLUTE NECESSITY to generate revenue for your bosses.
I wish I was surprised that you don't see the difference.
Wait... WHAT?
Just because they're "fantastic" they can get away with signing a contract and not honoring it? What planet are you living in??
Legal obligation
These guys know the drill, you sign a guaranteed contract, you may be get traded. That's the business. Just deal with it. Don't like it? Then don't sign the contract in the first place. It's not about them being "thugs" or not. Signing a contract means you are obligated - thugs or otherwise.
"Absolute necessity to generate revenue"
C'mon we all know the reason these guys have reached this level of popularity is because of the marketing machine that is the NBA in the first place. It starts from the moment they're drafted, the BIG event, there's cameras following them everywhere, ESPN, etc, etc, etc.
If the NBA were to start afresh with a set of rules + new system + new crop of rookies plus some D-league players this year - OK it may take 5-10 years to get up to the same level of competitiveness. But I bet with the likes of Barnes, Sullinger, Rivers, etc coming through each year the league would be up to a certain level of competition in not too long and these players will become household names - while the likes of Kobe/Paul, etc, will be riding buses in Europe/China if they decide not to join.
Remember a few years back when the refs were on strike and the NBA quickly trialled a new batch of "newbie refs"? The old refs quickly caved cos they know that if they don't, eventually these "newbie refs" would take their jobs.
"Fantastic" is relative
Imagine if the NBA start with a brand new fresh crop of players as stated above. With clever marketing and a 24/7 ESPN coverage, these new guys (say, Barnes, Sullinger, Rivers, Davis) will quickly become household name. And in 5 years time? We'll be saying "Kobe who?".
Start your own league
Yeah - why not?? If they hate the system so much. If they're not OK with the "business" side of the business. I don't see them doing that. Why? Because they know that even if they have their own league, the same issues will arise. Except the players will be arguing amongst themselves instead of to the owners.
New CBA
There's a reason the players got shafted in the new CBA. The players need the NBA - not the other way round.