[michaelt]

  It feels like this hasn't been done due
  to lack of desire for it rather than
  any inability to execute.

I always assumed it was three reasons:

1. A chicken-and-egg problem, where users don't join micropayment platforms because they don't have major content producers, and content producers don't join platforms because they don't have users.

2. There are actually a bunch of different visions for micropayments (articles costing $0.50 vs $0.05 vs $0.005 vs $0.0005; voluntary vs paywalls; drm-free vs drm; ad-free vs ads vs ad-blockers; articles vs music vs video; automatic vs manual payment vs automatic-with-refunds; quality professional journalism vs anyone can take part; free speech vs not funding hate groups....) and as your product vision becomes clearer, more and more stakeholders notice your vision isn't quite their vision.

3. If you're making a stored-value account in my country there's a bunch of regulation due to a history of scams. Presumably you would have to comply with regulations in every country you operate in, which would be nontrivial.