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.