It’s because the taxpayers in Massachusetts (like in many other states) give 25% back on every dollar a movie spends in the state.
No, this is not a good deal for the State and no it does not build lasting infrastructure. The money flows almost entirely out of state.
At best the incentive creates a cottage industry that will immediately leave if the incentive is ever repealed.
Source: handling these tax credits is my fiat mining job and BOY IS IT A FIAT JOB