Why cannot rates be expressed as a percentage?

It depends on the context. If the two quantities being compared (numerator and denominator) are of the same kind then the rate CAN be expressed as a percentage. For example, both are money and the rate of interest is usually expressed as a percentage.

However, the Word "rate" is often used for comparisons over time. For example, speed is the rate of change of location (or distance) over time. Here the two measures - distance and time - are different and so the answer cannot be expressed as a percentage.