Why isn't there a driver's license that doesn't allow passengers?

If you need an excuse to tell someone "no" then you have serious issues to deal with in your life and I would seriously question if you should be allowed to operate a multi ton machine that contains many liters of explosive liquid and/or highly flammable batteries in a shared environment. While driving is a very common activity it's quite dangerous and akin to operating heavy machinery. You need both emotional and physical maturity to do it. While the physical maturity is usually measured in minimum years, we don't have a great way of measuring emotional maturity.

Quite honestly your statement scares the living shit out of me that I might be sharing the road with someone who's incapable of asserting their own wishes.

To ignore your statement and focus on your question, this does exist. In the United States is common for people to get a license at a young age. Usually 16 years old. Some states use a graduated licensing method where in the beginning you can't have passengers unless they are adult, licensed drivers and you can only drive under special conditions. This allows new drivers to gain skills without distractions of their peers.

The idea of creating a license category because you can't tell your mom "no" is a bizarre as China freaking out because an unknown and unmanned airship was shot down because it violated sovereign air space.

I guess this is what's meant by the new normal.

In fact, there are drivers' licences which require a more experienced driver as a passenger in some European countries.

Eg. new drivers in Serbia can only drive cars with 80kW or more powerful engines if they have a passenger with 5+ years of driving experience (well, they hold a licence for that long).

Yes, they are obviously solving for a different problem.

If you are unable to build up social manners to reject people you don't feel like driving, you can always get a two seater and pull the passenger seat out.

Why isn't there a driver's license that doesn't allow passengers?

There is not enough demand for such a thing. If one is unable to say no, the other option would be to get a vehicle that only facilitates one occupant. e.g. motorcycle, moped, bicycle, e-bike, tractor, small street legal side-by-side, Japanese mini-truck if your state allows them, utility truck with equipment occupying the one passenger seat, etc...

There should be a license that requires more than 1 person in the car during most of time. Seems ridiculous to have a car sized space just for one person. Make cars the size of bikes first.