As Walker understood that he was perceiving more, he was focusing on these new perceptions. The idea that he couldn't influence anything here without some form of understanding of death mana was hard to grasp.
Simply because he would need to also use his own mana to do so. It wasn't that the herbs and anything else here weren't possible to touch, it was just that they had rules.
They still had physical form, but it was like trying to touch something surrounded by illusions. It was hard to gauge where it was or how close it was. More like impaired vision of someone were to compare it properly.
Beyond that, it was also the fact that there were plenty of other things with similar requirements. Herbs that would severely burn someone if they weren't handled with fire mana. Wind repeated monsters that couldn't be touched since they were mostly made of wind or air itself.
