Different plans.
Lakers & Knicks choose the free agency to get better each year because they believe with their big market they can attract anyone. It didn't work well this season because they were too bad but I'm sure next year it will start to be better for them.
As for Boston, Ainge believe that the hunt for a star via trade is the right plan.
Considering that FA won't come in Boston unless you are already contending, I think this is our best plan. So Ainge is collecting assets, hoping that when a Harden, Cousins, Durant... is available via trade he will have the right assets to deal with. Meanwhile, Ainge is hoping one of our young is a gem. With all these picks from last 2 seasons and the next one, probability will tell you there is a good chance we'll have a diamond in the raw.
We are already in mediocrity, no point to go deeper into mediocrity and sacrifice flexibility right now.
Different stories, can't compare.