Simply because of how we treat omnipresence.
Sometimes verses have Omnipresent Characters, and those verses may also have realms made up of information or data, but we don't give them Abstract Existence for Information & Data because of that.
Edit: It's the same way we don't give beings stated to be Omnipotent in their verses, every ability demonstrated.
The issue here is not that, but rather that this is a different case altogether and actually qualifies as a feat.
Infinite Zamasu ultimately merged with everything within the Super Timeline, including the Super Timeline itself, which granted him 6D HDE and AC1. However, he should also possess NEP1 and BDE1 as well, since he merged with the voids too, and those voids possess a NEP1 and BDE1 nature. This is a feat and should be treated as such.
To clarify, I am not saying that his entire being would possess NEP1 and BDE1 simply because he became existence itself. Rather, the NEP1 and BDE1 aspects would only apply to the specific part of him that merged with the voids, not to his whole existence. Only the “void portion” would have those properties, because Zamasu merged with literally everything.
For example, he currently has 6D HDE because he also merged with the Super Timeline, yet his existence within three-dimensional and four-dimensional space is still vulnerable to 2-C to 2-A level characters, since that part of his existence falls within their range. They can destroy Zamasu across the entire 4D range, but they simply cannot affect the 6D extent of his existence. The same logic applies here: such entities would not be able to interact with the portion of Zamasu that possesses NEP1 and BDE1 through merging with those voids. In other words, they cannot affect that specific part unless they can interact with NEP1 and BDE1 in the first place.
In short, permanently and completely erasing Zamasu would require an entity capable of interacting with his 6D HDE, BDE1, NEP1, aspects, and AC1. That is necessary in order to truly eliminate him.
Zamasu should possess NEP1 and BDE1 only in the section of himself that merged with the voids, not throughout his entire being. That is the point I am trying to make, and it does not contradict him possessing all of these properties simultaneously, because Zamasu is a special case here and the situation can be organized through the nature of his merger.