Remember: language always has a social context. Always.
- Why do languages borrow words and phrases?
- Sometimes: consciously, to fill in a gap in the language, by bilinguals who care about the target language. That takes work.
- Rather more often: as a transferral of prestige and connotations from the source language, by bilinguals who want to keep thinking in terms of the source language. That takes less work.
- And remember that French was more prestigious than English in scholarship and culture for a very long time. Not just during the middle ages, but up to the 19th century; the phrases OP quotes are not mediaeval.
- Why do languages not borrow words and phrases?
- Sometimes: because they want to keep their elite register accessible to monolinguals. That takes work.
- Rather more often: because they are motivated by an ideology of linguistic purism, which objects to interference from other languages. That also takes work.
- I like purism in the abstract, and (unlike many an English-speaker) I do not believe that it is a bad thing in principle. But purism does not have a great track record in English, in particular: Has there ever been an attempt to “purify” English by removing Latin/French words and reintroucing the old Germanic words (like many languages did)?
- Why would languages get rid of old foreign words or phrases, and adopt native phrasings instead?
- Drop in education and exposure to the source foreign languages. That’s certainly been at work in English: you no longer have to learn Latin to be educated, and French has been marginalised in the US if not the UK. But that drop is symptomatic of other causes.
- Drop in relative prestige of source languages against the target language with the language community. Well that’s certainly happened in the English speaking world in general. And accordingly, there’s very little Graeco-Latin in computer science, as a new discipline. But be careful: we’re talking about specific educated registers of English, not English in general, or even educated English in general.
- Why would languages not get rid of old foreign words or phrases, and adopt native phrasings instead?
- Inertia. Never underestimate the role of inertia. It explains a lot of craziness in language; such as English spelling. Getting rid of well-established old foreign expressions takes work. As you’ll have gathered, the reason those old foreign expressions took root in the first place was that it took less work. And a lot of Latin and French has dropped out of English because it’s more work to maintain them. But expressions of the kind OP is bringing up are so entrenched in their registers, that it’s more work to get rid of them.
- Purism? Not a trait of the English language community.
- Making it easier for EFL learners? Not the way English works (currently): native speakers still set the norms for English. (Despite the fact that alphabet is being used a lot on Quora to mean letter.)
Language as it is practiced does not line up according to what is rational. It is about what is most aligned to the speakers’ ideology, and what represents the least imposition to speakers.
I’ll add that the threshold for words is much higher than phrases. Coup is no longer regarded as French, even though coup d’état might be. Neither is sortie. You’re left with raison d’être. And that’s not a commonplace expression.