Because there may be more teams with full coffers. I would have looked at it 1-2 years ago... I've consistently been in the corner of "you'd rather trade a guy a year early than a year late". Which contender has more to offer than Miami? Typically only contenders bid on players in their 30s on huge contracts. To expect a Gobert return (it was an outlier) or a Donovan return is a bit optimistic given the market is turning and bunch of teams have already pushed in. Even if you compare Herro, 3 firsts, and Jacquez/Jovic... its not far off the Donovan trade. Lauri blew up but if he had just been Cleveland Lauri then how far off is the best Miami package?
If you want to maximize the return on a Lillard deal you trade him when there are contenders with a bunch of assets ready to bid... the market is not frothy right now. The Suns for example after the KD thing falls through... If Portland says 4 firsts and swaps plus CP3 for Lillard you think Dame makes a stink?