As mentioned by a sibling comment, fMRI have a sufficient spatial resolution (i.e. you can watch the activity of a few number of neuron at a time instead of grouping them by batch of hhundreds of them), but the temporal resolution is low (order of magnitude is seconds; a single action potential is 5ms).

Here's a graph that summarize the temporal/spatial tradeoff of the different BMI tech: https://www.researchgate.net/figure/The-various-neural-recor...