I'll focus on Latin because I don't know how much this applies to Greek, Sumerian, Sanskrit, Akkadian etc.

Lots of translators focus too much on individual words, and miss the text. So when handling Latin they

• spam less common synonyms (specially Latin borrowings)
• try to follow Latin syntax too closely into English
• use large sentences full of appositions

Less common words, fancy syntax, large sentences? That makes the text sound old timey.

I'll give you a practical example with Caesar's De Bello Gallico. Granted, the translation is from the 1800s, but even for those times it's convoluted:

[Original] Gallia est omnis divisa in partes tres, quarum unam incolunt Belgae, aliam Aquitani, tertiam qui ipsorum lingua Celtae, nostra Galli appellantur.

[Bohn and McDevitte] All Gaul is divided into three parts, one of which the Belgae inhabit, the Aquitani another, those who in their own language are called Celts, in our Gauls, the third.

There's almost a 1:1 word correspondence. With the following exceptions:

• "the" - because not using it in English makes the text sound broken
• "in", "of" - because English demands prepositions more frequently than Latin
• "their own" - because English lacks a 1-word equivalent for "ipsorum"

For reference here's how I'd translate the same excerpt:

Gaul is split into three parts. One is inhabited by the Belgae; another, by the Aquitani; the third one, by those who call themselves "Celts", and that we call "Gauls".

I'm not a good translator, mind you. And I'm myself fairly pedantic. Even then, I believe that it delivers the point better - it's streamlined, using concise and clear language, like a military commentary written by a general is supposed to be. But it is not a 1:1 like those guys obsess over.

I can think of a few reasons a translator might choose to do that:

• The original author was using language that was old-fashioned in their time (e.g., a medieval Latin writer imitating Cicero, or a Hellenistic Greek writer imitating Thucydides)

• The work in question had its greatest historical impact long after its original composition, so its language would have seemed archaic to the relevant readers (e.g., the Vedas, Avestas or Analects)

• The translator is trying to maintain consistency with canonical translations of related works done long ago (e.g., translating early Christian writings in the King James style)

• The translator wants to create a general sense of cultural distance, if placing the culture of the original work in a modern context would be misleading

That might have been the style it was translated at the time and no one updated it.

I think all the other comments have some validity. In particular, the most famous or readily available (and certainly the most readily available for free) translations often were done by a "classy Englishman from the 1800s." In the case of Latin and Greek, sometimes it was the 1700s.

Another point not yet raised is that the languages will simply convey different things in a single word or phrase, and if you want to maintain that meaning, sometimes tone will take a backseat to linguistic data. Good translations will provide context for what the tone and intended audience would be, as far as we can tell, and also for the translator's methodologies and biases.

This introduction to a modern translation of Cicero gives, I think, a fairly decent rundown of the difficulties inherent in this process. See page 7; it is a Google Books link, fair warning.

I know from Historia Civilis that at least part of it is with the intention of preventing the masses from knowing that oh so noble and dignified Marc Anthony could swear and curse like a sailor with chronic blue balls in his letters to Octavian

Literally one of his letters is complaining about why Octavian is making a stink about him fucking Cleopatra when he's already plowed his way through Rome's finest bachelorettes, who he lists by name!

A lot of old stuff doesn't translate cleanly, even between two modern, similar languages.

Imagine if the only German you knew of was from an 80 year old German-English dictionary that you had to use to translate a news article written in German last week. Newer words are completely absent, places have changed names, good luck pronouncing anything correctly, and idioms, slang, and many scientific, political, and cultural reference are completely indecipherable. Proper nouns (ie names) will be somewhat challenging, especially names invented in the last few decades.

I imagine with a lot of ancient languages there's about 3 people capable of translating it and they're all university professors in ancient history, who write like that normally.

Language changes over time so they're probably trying to make the translation sound accurate to the time period it was written in I would guess. The tone of the text is sometimes as important as the actual content

Do you have an example?

Could you give some examples of what you mean?

If we're talking about English, there were different kinds of English which were spoken (Old English, Middle English, Early Modern English, and Modern English. And no, Old English doesn't just mean English that sounds 'old'. It was an early variety of English that was spoken from the 5th/7th century to 1066). All of them had their own phonology, morphology and syntax. In short, they followed their own linguistic rules and conventions. The way they sounded wasn't just randomly made up. To find out more, look up the varieties of English I mentioned or take a look at this Wikipedia article about Historical linguistics.