It's a few things.
To get dumpoffs for bigs, first you need penetration. The only guy we have right now who can regularly penetrate, and penetrates to create in Reke. Brooks plays outside, comes off screens firing, and rarely gets into the teeth of the defense. IT is not penetrating with much success this year, and when he does he's looking more for his own shot and unfortunately turning the ball over at an increasing rate when penetrating into traffic. Jimmer has had a few nice dump offs, but he too is coming off screens looking to shoot or looking for his runner in the paint. MT is MT.
We also don't have a resemblance of a motion offense, or really any offense, which makes it easier for the guards to get inside off movement and on the ball screens. If we're iso'ing Brooks, Jimmer or IT up top and expecting them to break their man down, draw help in the paint and create opportunities for our bigs, then we'll be disappointed with the results. If these guys were constantly curling off screens and there was more movement all together, forcing the defense to make decisions, help, switch, that's where gaps are opened up. That of course would require running an offense which we appear to not have. Even when Reke does get into the teeth of the defense, it's not usually because Smart ran something to make it easier for him but rather it's based off Reke's natural talent, and we don't have another player with the natural talent to just beat his guy 1v1 and cause the defense to collapse.
Dumpoffs are created when you force the defense to react, to move, to shift, to make decisions, and the easiest offense to defend is what we run, an iso heavy offense with little movement. Guys just sag off our players in help position and we're not doing anything to cause defenses to have to make decisions.
Then there's the aspect that Cuz prefers to stay outside. He's lining up for jumpers most of the time, and prefers a pick and pop or spotting up 15 ft away than rolling hard to the paint. Only success we've really had with getting Cuz on the move and hitting him for a dunk/layup is when he and Reke run the two man game, but Smart seems intent on not allowing them to do it that much.
JT gets himself into good positions for dumpoffs, and is probably our best big at finding that gap in the defense looking for a dump off. Chuck is slow and short. Even when he does get a dump off he has a tough time converting given he's the height of most SG's and has the same vertical as my grandma.
So really, it's lack of penetration from anyone other than Reke looking to create, bigs who prefer staying outside or are undersized inside, and lack of an offense with movement which creates openings/lanes by forcing the defense to move and make decisions.