Step-by-step explanation:
because speed is distance per time interval.
we cannot meassure a precise speed at a single point, because then distance would be 0 and the time interval would be 0, and we would get 0/0, which could be anything and is therefore undefined.
we can split a journey into smaller segments and calculate their individual speeds, but these are then the average speeds per segment.
so, we never can express speed differently than as average speed for a given distance in a a given time interval.