Inputs spending outputs with the same locking script repeat some of the input elements necessary to spend the input. E.g. a P2PKH input needs to reveal the public key corresponding to the public key hash appearing in the corresponding transaction output’s output script. This would need to happen again if someone spends another output with the same output script.

If we made reusing the same address cheaper that would undermine the privacy best-practice of using a new output script for every payment. I therefore see zero chance of such a change being adopted in Bitcoin.

Every signature will be different as data to be signed in each input will be different, that is what matters.

Here's random example transaction I just found, it spends exact amounts from multiple inputs, sent to the same address. https://mempool.space/tx/466ddadf42d82027411b2984a6b1b0f1d9631fb05f6e61d97e7634bf34119f2e

Click on "Details" there and compare "Witness" field of each input.