Why did English borrow verbs ending in -ish? [closed]

Why did English borrow verbs ending in -ish, but not in anything else, from French?
This seems quite obscure because it didn't import the verbs from the infinitive French forms, but through some sorts of obscure stems. I mean wouldn't it be more straightforward to use finir than finish?
As the etymology section of Wiktionary suggests, -ish comes from some sort of obscure French stem -iss. English borrowed change from Old French changier, why didn't it use the same deriving method with -ish verbs?
Dropping the -r, adding the -en, then dropping it again in Modern English. I'm not sure what that'd make an English verb from finir look like, but it seems like a more natural and straightforward route.

There are a number of verbs ending -ir in modern French, where the corresponding English forms end with -ish. Some of them are établir, finir, nourrir, polir, punir. These are all conjugated the same way, so I'll just use finir as an example.

In modern French: finir is conjugated je finis, tu finis, il finit, nous finissons, vous finissez, ils finissent. So every form except 3rd person singular has an "s", and all these "s"s used to be pronounced. Borrowing the form finis rather than finir is quite understandable, since it probably occurs just as often in ordinary conversation. And having /s/ turn to /sh/ is a very common sound change in many languages.

What I don't know is whether the change from /s/ to /sh/ happened in Middle English or in Anglo-Norman French. (It didn't happen in the French currently spoken in Paris.) Maybe a linguist could tell us.