Last 10 star trades:
Harden (to Clips), Jrue, Dame, CP3, Beal, KD, Kyrie, Don, Gobert, Harden (to Philly)
Every deal had servicable players going back to seller (including two since turned stars Bridges and Lauri). And none of them were even heavy on expiring salaries as far as I remember.
Your logic might say "teams want expiring" but in reality they dont. They want good players/prospects in addition to picks.
Harden - Expiring salaries were a priority
Beal - Washington got expiring salaries. The re-routed CP for Poole and assets.
Jrue - Not expirings but 2 two year deals for decent players (Boston also had no expirings).
Dame - Jrue (an expiring) and picks.
KD - Great young players (worth multiple picks by themselves) and picks.
Kyrie - DFS and Dinwiddie both players that would have positive value by themselves. DFS would have gotten a first for sure.
Don - Lauri/Sexton picks Ochai and picks
Rudy - Expirings and low salary guys plus picks.
Harden - BS (who had value at the time individually) and picks.
If you are going to use absolutes like "none" please do your homework. At least 4 relied on salary flexibility and expiring contracts. Two of them (Boston/Cleveland) didn't have expiring salary to match. The other players involved in trades for the most part had serious trade value on their own - Bridges, Ben Simmons, DFS, Cam Johnson. The Brooklyn trades are unique in that their draft picks belong to another team so they wanted good (not serviceable) players as well.
We know exactly what Collins value was this summer. Essentially a second round pick and some unsavory 1 year salary. He is not equal to any of those guys outside of maybe Sexton.
So its not my logic... its history... and history states that most teams seek financial flexibility when trading off a star (in addition to draft assets and young players). There are outliers but even in the outlier situations we could easily substitute expiring salary for Collins and would have more flexibility with the cap and still accomplish the goal. It isn't like Portland was sitting there saying "we can do a deal with someone else for an expiring contract and the same picks as Boston is offering... but man we really need to get Malcolm Brogdon and Robert Williams!"