Here's why:
* "Worse" needs a comparison. "Worse" implies a comparison between two things. You need to specify what is being compared to something else.
* Incorrect word order. "Not that worse" is not a standard English phrase.
Here are some examples of how to use "worse" correctly:
* "That's worse than yesterday." (Comparing the situation to something else)
* "This situation is worse than I expected." (Comparing the situation to a previous expectation)
* "The pain is worse now than it was before." (Comparing the pain to a previous state)
Instead of "is not that worse," you could use:
* "Is it not that bad?"
* "Is that not bad?"
* "Isn't that bad?"
These options all correctly express the idea of something being undesirable or unpleasant.