Assuming by "they" you mean the Romans who spoke Latin, they did in fact turn numerus into a verb: numero, numerare.
nŭmĕro , āvi, ātum, 1, v. a. numerus,
I. to count, reckon, number (syn. recenseo).
I. Lit.: "si singulos numeremus in singulas (civitates)," Cic. Rep. 3, 4, 7: ea, si ex reis numeres, innumerabilia sunt;
In English, we also borrowed a prefixed form directly with enumerate, from enumerare.
ē-nŭmĕro , āvi, ātum, 1, v. a.,
I.to reckon up, count over, count out (class.).
I. In gen.: “jamne enumerasti id, quod ad te rediturum putes?” Ter. Ad. 2, 2, 28: dies, * Caes. B. C. 3, 105, 2: “peculium,” i. e. to rate, estimate, Plaut. As. 2, 4, 91: “pretium,” to count out, to pay, Cic. Rosc. Am. 46, 133. —
French, too, has énumérer.
Regardless of the facts, the question shows a misunderstanding of how languages work. There is zero reason for a language to form a verb from a particular noun. Just because Turkish and Latin can do this, doesn't mean they have to. It makes no sense to ask "why" a language does or doesn't do something, as they answer is always plainly "just because." You could turn the question on its head and ask "Why does Turkish not have a separate word for counting that doesn't derive from number?" There's no logical reason for any given way. Words are because simply and only because people use them in that way.