Given that the longbow was so devastating

I'm afraid you're starting from a rather inflated premise here. The longbow is certainly an important part of the English tactical system of the HYW, but it is only part, and not the entirety of the consideration. The Medieval Warfare section of the FAQ has a subsection on longbow matters, which I highly commend to your attention, in particular the one about Milanese armour. To these, I'll also add a writeup on what the French did do to counter English longbow spam, by u/Hergrim.

why didn't the French do whatever they could to also have it?

They did have it, and they tried to have it in larger numbers and in better-trained hands, but with limited success. See the discussion by u/Hergrim and I in https://www.reddit.com/r/AskHistorians/comments/7s63bl/if_the_english_longbow_was_supposedly_a_better/

Given that the longbow was so devastating,

That the French tried to have more longbows shows that they appreciated its effectiveness, That they didn't try harder shows that it wasn't as devastating as popular culture would sometimes have it (e.g., arrows going through armour as if it was tissue paper). That the French won the Hundred Years War shows that they didn't need to use the longbow in large numbers to win.