Because then you have to slow down to keep a safe following distance again. Then it happens again, and again, and again.

One car driving significantly slower than the rest of traffic can itself be a safety problem:

"To keep a smooth traffic flow, some highways also have minimum speed limits. If you drive slower than the minimum speed you can halt the traffic flow and create a dangerous condition. Even if there is no minimum speed limit, those driving too slow can be as dangerous as those who drive too fast."

https://dmv.ny.gov/about-dmv/chapter-8-defensive-driving#yre...

You don't need to drive significantly slower than others, just slightly slower so a new gap appears in front of you.

The conversation you are in:

1. It's the fault of the driver behind, for not leaving space

2. It's impossible to leave space, because cars will move in if there is a large gap

3. Why is this a problem?

It is not impossible to leave space. Granted, you need to do it every time a car moves in front of you. But such is life.