>As is often the case with tech companies, new business models, like tipping in this case, play on ancient ideas and practices, but make them many orders of magnitude more accessible and scalable through software.

The scale is the very thing that kills this model. With a small, close community, there is huge societal pressure to live up to your obligations, and not doing so has huge social costs to you (you become isolated from your community). At Internet scale, you no longer have the community to enforce social norms, and not living up to your obligations may actually be lauded in r/lifehacks.