Why does Feistel encryption algorithm encode half block every time?
You can't.  Some of the block is used as the input to the F function, and the rest is modified. Those can't overlap.  If you wanted to modify all of the block, there wouldn't be any input to the F function.

You might be interested in https://en.wikipedia.org/wiki/Feistel_cipher#Unbalanced_Feistel_cipher