GitHub's incentives and design constraints are different than ours. GitHub needs to offer something that covers a very large user-base, to cover the widest possible number of workflows, and they've done this by offering basic ephemeral VMs on-demand. CI and builds are also not GitHub's primary focus as an org.
We're trying to be the absolute fastest place to build software, with a deep focus on achieving maximum performance and reducing build time as much as possible (even to 0 with caching). Software builds today are often wildly inefficient, and I personally believe there's an opportunity to do for build compute what has been done for application compute over the last 10 years.
GitHub Actions workflows are more of an "input" for us then (similar to how container image builds have been), with the goal of adding more input types over time and applying the same core tech to all of them.