Why do the Suns do this?

The team is going nowhere, you can't retool to become a contender for as long as Beal is on the payroll and no one is trading for that contract. If you're Pheonix and take loyalty to Booker out of it, there is no point sitting around the play-in through the entirety of Book's prime waiting for Beal to go, you can get great value for him right now and bring the possibility of a ring much closer in the timeline.

You're getting 5 first-round picks to kick off the rebuild, with two of them being lottery picks in the upcoming draft which is looking strong. Jalen Green becomes the face of a younger Pheonix side that has much greater depth and allows for you to get off all of your bad contracts without becoming a lottery team in the short term. The starting five would look something like this:

Beal Green Vassell Smith Jr. Richards

You can focus on building a more athletic and well-rounded team, shifting your negative assets and opening up cap space for when that Beal contract expires. You can take advantage of having this deep rotatino to figure out who you want to build around long term, and can give plenty of minutes to rookies/sophomores to build their value. Two lottery picks in the upcoming draft are huge with how strong it is looking, you even may be able to get your hands on a Khaman Malauch to challenge Nick Richards and Oso Ighodaro.

Why do the Rockets do this?

You get Devin Booker.

Adding a top 20 player to a side full of young talent turns the Rockets into instant contenders with a wide championship window. You upgrade on Jalen Green whilst only giving up a young guard who is struggling for minutes to develop (Sheppard) and a Jabari Smith Jr. who you would have to pay in the near future. Only losing two first-round picks in this deal means there are still many opportunities over the next 5+ years to pick up talented role players on cheaper contracts through the draft, to fill out a starting 5 that would look something like this:

Thompson Booker Brooks Eason Sengun

Why do the Spurs do this?

Bringing in KD turns this team into an instant contender in the west, and you don't have to compromise a lot in the long term. Losing Vassell and Johnson as well as three first-round picks is not an awful lot to give Wemby his first run at a ring. Take advantage of his and Castle's rookie contracts pre-extension and you can compete right now, have a down year or two and then be right back up there with the 2027/28/29 draft picks you still have. Durant raises the ceiling of this side and allows for you to push for a ring in bursts as opposed to just being a good but not elite team in a competitive west consistently across say 5 years. A starting five that would shoot to the top in terms of quality in the association would look like this:

Fox Castle Durant Sochan/Barnes Wembanyama