Higher framerate = more frames per second = more frames overall = bigger file, so there is framerate capping put in place by video recorders.
In short, an uncompressed 300FPS AVI file would take up a ton of space. Editors that are targeted for people without a business mostly cap out at 120 FPS, anyway.
Why would you want a 300FPS video? I don't even think YouTube supports anything over 60FPS.