The question is rather: why not? They are good words! Even though the English language was influenced first by Norse, brought to the island by the vikings, and later by French that came with William the Conquerer, both "waves" weren't strong enough to erase certain parts of the "original" English language, brought to the country by the invading Angles, Jutes and Saxons in the 400s. People kept using those words that were useful and fitting in their daily lives. People tend to not like change and only tamper with things that don't work anymore. And lots of words from middle or old English seem to still work fine.