Answering phone calls with side button ?

Hello,

This question may have been asked before, but I couldn't find a satisfactory answer:

Just wondering why one cannot answer a phone call directly with any of the side buttons on an Iphone, not even by way of an option to toggle on/off.

There are ways to use Siri or the automatic answering options, but it seems impossible to answer but just pressing any of the side buttons (power on/off nor the up volume button).

There must be a good reason, which I would like to know.

Thanking you in advance for providing me with possible explanations as to why this is so.