It's not? The second definition for the word in Lewis and Short, after "a horseman" is this:

Equites, the order of knights, the Equites

And you'll find that when it's translated at all by classicists (which is rare these days--the differences between the equites and medieval knights has caused it to fall out of fashion. Every classicist knows what equites means in any case, so why bother to translate it?) it's translated as "knights," with no Roman qualifier at all. I don't recall ever seeing a work of classical scholarship that translated the word systematically as "Roman knights"--that just wouldn't make sense, who else's knights would we be talking about in a work about the Romans?

In any case, the equites aren't called knights very often these days, because they really don't have much to do with medieval knights, the only kind of real knights. The equites were a social class, originally informal but formalized during the Principate, composed of plebeian men who fulfilled the property requirements required for entry into the senate but who were not senators or of senatorial rank. Since entry into the senate only required the holding of magistracy above the rank of quaestor these were men who had no political ambitions and whose families had no political ambitions, and had therefore stayed out of direct political affairs. Quite different from the knights of the Middle Ages