Why can’t I purchase AppleCare after my warranty is over

purchasing applecare after the normal warranty has expired would be the same thing as buying life insurance after one got sick, if you could nobody in the right mind would purchase it prior

you can advice apple to change their position using their feedback channel

Product Feedback - Apple

All warranties and insurance must always be purchased before an issue occurs. And after some period of time (a year in the case of an iPhone) nobody will warranty or re-warranty an older device.

I don’t understand why you’d be surprised by that - it’s the same situation with any and all consumer products.