They're charging you for orchestration, log storage, artifact storage, continued development of the runner binary itself and features available to self-hosted machines. What would your own machine do without the runner and service it connects to?

They still have to manage state between their servers and yours.

I'll be investigating gitlab tomorrow

Have used all of the big 4 forges in anger over the last decade. GitLab isn't perfect, but I'd take it over GitHub any day of the week.

it charges you to use the platform features that enable your use of self-hosted runners

For the same reason they charge you for running Word, even though you're the one who has to write, I guess?