I think this is because all frontier AI providers integrate with GitHub as a default git forge through their GitHub apps platform and their free tier is too generous. They should either charge some of these huge partners access to GitHub apps or partition the free tier off and have a different SLA.
If the frontier AI used GitLab as a default I'm sure they'd be the ones suffering now.
The frontier AI (OpenAI, Anthropic, Cursor, Mistral, xAI, Perplexity plus more) all run CI on Buildkite, without the suffering.