16MHz isn't a power of two either (that would be 16 777 216 Hz)
The problem lies in the fact that the common baud rates (100, 300, 1200, 2400, 4800, 9600, 19 200, 38 400) aren't simple multiples or powers of 2 either, and is further compounded by the fact that humans like decimal time periods.
However, digital logic doesn't really work out easily for odd divisors, and prefers powers of two.