I note that both Airbnb and Uber marketed as "use part of something you're not otherwise using", and almost immediately became professionalized. Full time drivers. People buying apartments to let out.
Maybe they wouldn't have worked without that professionalization? Which is of course not possible if you're going the "passing traveller" model.
This is the key thing. None of this “trust a stranger” stuff actually works out. Uber isn’t actually a rideshare. It’s a professional driver. Airbnb isn’t a room in someone’s house. It’s an apartment rental. GrubHub isn’t someone who picks up your noodles when they pick up theirs. It’s their job.
The courier model could totally work the same way. You want someone to drive your package from San Francisco to New York? Someone will happily do that. The trick is they will want to get paid. No one’s doing this stuff basically for free as a favor or to help OP’s company show a profit.
Maybe worked but at very small scale. The early Lyft with fist bumps and much more casual driver interactions worked at some level but was pretty small--and I actively avoided because of the vibe. You may borrow a tool from a neighbor but it's not a routine or neighborhood-wide thing for the most part.