Spending decisions aren't always made by penny-pinchers. Sometimes it's inertia, not disrupting the status quo, dance with who brung ya, avoiding work, a hundred reasons why groups of people transition from a free plan to a paid plan with the same company over time if the company hasn't given them a sudden, dramatic, stressful reminder that they need to shop around and always be ready to switch to whoever gives them the best deal.

Docker wants to be a sticky default, not a commodity that has to compete with ECR and GCR on cost and features.