If atmas impaler granted scaling health per attack damage instead

It would see more usage and here is why:

Base champion health on average quadruples over the course of a game, not counting itemization. This means that the bonus attack damage per health granted by atmas impaler is strongly skewed to be more powerful per champion level, which severely restricts how powerful it can be as an item bought in the earlier levels.

Base champion ad on average approximately doubles over the course of a game, not counting itemization. That means that the item's power as something that can be bought in the earlier levels would not be nearly as restricted if it granted health per ad rather than ad per health.

Furthermore, there are many better hybrid and pure defensive options available to champions who would favor the current atmas impaler, which does not do any specific job better than the alternatives. This change would cause atma to atleast make it a favorable choice for high attack damage assassins who want some more durability vs physical damage without devoting an entire item slot to a pure defensive function. Much of the value of the item is tied up in the critical strike chance, which would require the assassin a longer window of engagement to access reliably, which is a fair trade off for the free durability.