Patients undergoing brain surgeries are routinely asked to volunteer for neuroscientific studies and they often do so. Terminally ill patients also donate their brain tissue for tons of studies more often than you would think. But the scope of experiments that one can do with them and the available multimodal data available from parallel studies in human cortex is extremely limited.
On the other hand, non-human primates are readily available in a controlled environment, can be used to generate huge amounts of data specific to a diverse set of tasks and experiments that you just can not do on the human patients.
The brain is a complex machine that is extremely hard to decipher just by studying a single species or one kind of experiments. The need for studies across species with a hundred different ways of collecting and analysing data is essential to first understand the mechnasims of the brain and only then (hopefully) be able to causally alter it for specific applications in medicine and so on.