Why do aliens in this star date 3834 carry knives?

CIRCUMCISION.

Knives are timeless. Knives don't need batteries. Knives are easy to hide.

Cuz it makes 'em look badass.

I think it might have been subtle nod from Rodenberry to some of the Arab cultures that carry ceremonial knives with them to this day.

If you mean in "Journey to Babel", the Andorian Thelev was actually an Orion, surgically altered to pass as an Andorian. His mission was commit murders and spread fear and mistrust among passengers and crew. That is why he carried a knife, to try to assassinate captain Kirk.

So I don't think there is much evidence in the episode about how often real Andorians carry knives.

Cultural reasons.