I had the same question — I understand that the Actions control plane has costs on self-hosted runners that GitHub would like to recoup, but those costs are fixed per-job. Charging by the minute for the user’s own resources gives the impression that GitHub is actually trying to disincentivize third-party runners.

Self-hosted runner regularly communicates with the control plane, and control plane also needs to keep track of job status, logs, job summaries, etc.

8h job is definitely more expensive to them than a 1 minute one, but I'd guess that the actual reason is that this way they earn more money, and dissuade users from using a third party service instead of their own runners.

Might be an estimation of logs storage/bandwidth.