Why can’t GibHub be reliable?

Good question.

Given that 100M+ users are able to run GitHub Actions (GA) for free, reliability is already out of the window and GitHub has objectively gotten worse in that since they got acquired by Microsoft and introduced GA.

Because it's not that easy and it costs money which they are likely no willing to spend. Also Microsoft took it over ~5 years ago an I guess we'll see the results of the different management style here.

Github's SLA is only for 3 9s of availability. For enterprises there is likely much more visibility for applications who's downtime had direct impact to revenue. If a system that processes orders or claims or shipments go down there is a direct line to $$$ lost. If Github goes down the impact isn't the same.