Here's why:

* PTC and thiourea are taste stimuli: These compounds are specifically designed to test the sensitivity of the taste receptors that detect bitterness. They are known for their strong bitter taste, and the ability to taste them varies significantly between individuals.

* Sodium benzoate is a preservative: While it can have a slightly sweet taste, it's primarily used as a food preservative to prevent spoilage. Its taste is generally not strong enough to be considered a specific taste stimulus like PTC or thiourea.

Therefore, comparing the ability to taste sodium benzoate to PTC or thiourea is not a valid comparison. The ability to taste PTC and thiourea is a genetic trait related to specific taste receptors. Sodium benzoate, on the other hand, is a preservative with a relatively weak and non-specific taste.

In short, while sodium benzoate can be tasted, it's not a taste stimulus that is typically tested for like PTC or thiourea.