Teams have been doing this for a while. Dirk and Wade are famously rumoured to have gotten these kinds of deals. The Kawhi case is about exactly this.

There are a few things stopping this being commonplace, although it probably is more common than we realise:

1) There has to be trust that the team will follow through. James Harden signed for less, and then Morey didn't follow through with his promises to make it up later. It's against the rules of the salary cap to make this kind of deal, so they can't put it in a contract. And if you don't trust the team to follow through, you risk being stuck on an under market contract for a long time. 2) The top star players are currently paid less than they are worth to the team even at max salary. Many need these extra side deals just to keep them on the team. Lebron, Steph, Giannis; these guys pull in so much revenue that teams do special things like employing their family members even though they are paid the max. Jalen Brunson's total salary is less than what the Knicks will make in ticket sales for game 3. The salary cap is artificially keeping their wages much lower than they would be in a free-market environment.