I would argue that there are many incentives to use platforms/apps for both the client and the provider/host, instead of contracting off-platform/off-app. For the provider, the app gives them 1) a steady stream of new leads and 2) it allows them to build up a reputation on the platform for repeat business. Maybe it is sometimes a bad sign to contract off-platform, because it could signal that they are not trustworthy enough, or do not have a high enough service quality and so would not have a high rating if they were on the platform? Also as the other user said: platforms often offer mediation/dispute resolution and insurance.

Platforms offer a way for individuals or small businesses to get started in a new sector without doing a lot of marketing for themselves. It's a bit like being part of an alliance.

I am not arguing for the continued domination and rent-seeking practices of Silicon Valley middle-men corporations. I'm arguing that the existence of any online bazaar/marketplace and the ability to search and compare, makes for excellent dynamics.

To go even further, I advocate for an emerging alternative that has been termed 'Protocol Cooperativism':

Instead of monopolistic rent-seeking platforms, it focuses on open source economic protocols that allow full negotiation between peer to peer users. Check out Ceptr.org, MetaCurrency and Holochain. It provides the frame and framework for ever evolving protocols (comprehensive versioning tech).