Because there's a finite bandwidth allocated to the FDR data system, and because it's a digital system (these days).
Because it's digital, it can only take data at discrete points in time; because there is a limited bandwidth, the points in time cannot be very frequent.
It's not uncommon to see quite important flight data parameters recorded at relatively low data rates - like one or two Hertz. Even something like normal acceleration, which we're discussing here, might only be at 8Hz. That means that any event lasting of the order of 0.1sec could be completely missed.
*In addition to the limitations on the (D)FDR side for data rates, you can run up against issues with your onboard computing/databus, if there's anything low rate in the line upstream of the FDR, it can end up basically filtering the data.
* What you're asking for is asynchronous data recording. That implies you have spare bandwidth to start with, or that you're going to discard something else to record the 'extra' data. Choosing what to discard would be troublesome.