Substantial revisions to clarify the post’s core claim, including but not limited to this summary at the end:
Effective Altruism claims that there is a large funding gap for cheap well-understood developing-world interventions.
Even the most aggressive plausible construal of this claim implies an annual funding gap that could be covered completely by existing major institutional donors.
If this is true, it implies opportunities for comparatively cheap experiments (relative to the endowments of major donors in the space) with extremely high information value.
Such experiments have not happened either because they are impossible, or because the relevant institutional donors think they have better things to do with their money.
Neither scenario suggests that small donors should try to fill this funding gap. If they trust big donors, they should just give to the big donors. If they don’t, why should they believe a story clearly meant to extract money from them?