Imagine if the guy we take at #9 ends up Thomas Robinson, Rozier ends up being the next Eric Bledsoe, and Brooklyn finishes bottom 5? Ainge looks like he should be fired on the spot.
Well, if that happened, the Celts would be bad, and get another high pick.
This rebuilding thing, it requires taking gambles.
Then we'd take another gamble, get another Thomas Robinson, and we end up becoming the next Minnesota Timberwolves or Charlotte Bobcats and sucking completely for the next 5 years with nothing to show for it.
The Knicks have Melo, the Lakers have Kobe. They will continue to bring fans in to games even if their draft picks end up crap.
Boston has a bunch of role players - they need a star, and they need one yesterday. They can't afford to take gamble after gamble the way teams like the Wolves can. Unless you're consistently getting picks in the top 3, your chance of picking up franchise-changing players in the draft is small.
Boston has a better chance by racking up large quantities of picks, rolling the dice on all of them, see what happens. If two of those guys end up decent players (in the Sully/Olynyk mould) and the other two end up complete garbage, then at the very least you have two young prospects who give you some form of solid trade value. If you do that two years in a row, then after the second year you probably have at least 3 or 4 such players. You then have some hope that you can use those guys in trades, along with other assets, to trade for a future star or a veteran who can help you win.
If you invest everything you have in one top 10 pick, and that guy busts, then you have nothing. Nobody is offering ANYTHING for Thomas Robinson. He's got not no value of any kind on the market.
The Lakers are probably actually LUCKY that Randle got injured last year, because nobody's seen him play so he still has some trade value on intrigue alone. Once he plays and everybody realises he's garbage, he'll be worth even less than Thomas Robinson - if they block a Cousins trade because they don't want to give up Randle they are total morons.
The ONLY way to build through the draft is to be bad enough to be able to get top 4 picks every year for about 3 or 4 years running. That way the odds even out, and you're likely to end up with one or two really good players at worst, and 3 or 4 good players at best. But to do that you need to be a GM for an organisation who doesn't care about losing horrendously for 4 years running (like Philly) - Boston ownership likely wouldn't accept that as a viable option.
So if you're stuck between choosing one guy in the 5-12 range each year for the next three years...or picking 4 guys later in the draft every year for the next three years...then both scenarios probably give you a similar chance of getting the type of player you are after.
As such, why bother doing the trade? May as well stay where you are.