Here's why:
* Meaning: "Sober" describes a state of being free from intoxication. It's a binary state: you either are or you are not sober.
* Usage: We use "sober" to indicate a person is not under the influence of alcohol or drugs.
If you want to express degrees of being free from intoxication, you might use other words like:
* Moderately/Slightly tipsy: This suggests someone is not fully sober but not heavily intoxicated either.
* Highly/Very intoxicated: This suggests someone is heavily under the influence.
* Perfectly sober: This emphasizes that someone is completely free from any intoxication.