"high rate of speed" or "high speed" to mean going fast

Why do reporters (and sometimes police officers) say that somebody was going at a high rate of speed when they actually mean high speed?

In physics, speed is already the rate of distance over time, otherwise known as velocity. Rate of speed is velocity over time, otherwise known as acceleration. By saying high rate of speed they would be implying picking up more speed.

Is there a social reason for using high rate of speed rather than high speed?

Because more words = more official-sounding. It's a bad phrase that has taken root in irrelevant situations. Adding "rate" adds nothing in most contexts.

However, the word rate also means "value" or "number". From Cambridge:

rate (MEASUREMENT)

noun

a measurement of the speed at which something happens or changes, or the number of times it happens or changes, within a particular period

The very first sense for the noun rate in Merriam-Webster is “reckoned value : valuation”. The word has more meanings than the one used in physics. One could therefore make the argument that a rate of speed is a speed which is reckoned (i.e. by measuring or reasoned estimation) rather than guessed.