To appease him and make him happy. I get it if he's a LeBron, KD, Giannis level type star, but he's a Robin not a Batman and maybe the Jazz feel they have no choice but to do that.
Even if he's likely to bolt before his contract expires. It is worth noting that if Donovan were to become all NBA his contract has a SuperMax escalator attached to it and the Jazz are the only team that can offer him that.