"assurance of its highest consideration" Why?

On diplomatic correspondence, it seems that EVERY country uses the phrase “The Embassy takes this opportunity to renew to the Ministry the assurance of its highest consideration

Is there any source for this? Why do they all say this?

It’s more conducive to cordial relations than “the Ambassador wiped his arse with your proposal and found it most suitable for this purpose”.

It sounds like it may be a translation of something which was originally in French.

And I beg to inform you that all languages were much more formal and stilted a century ago.

I remain your faithful and most humble servant. etc etc.

Diplomatic correspondence was, until relatively recently, conducted largely in French, regardless of the languages ordinarily used by the two countries concerned. And French letter-writing conventions retained a liking for elaborate formal openings and closings much longer than English letter-writing conventions did.