Cities charge a right-of-way fee, planning, and permitting process every time you connect to a public road. The county/city planning committee often requires new neighborhoods to cover the cost (often via HOA) of roads and their easements in the neighborhood. The end result is the neighborhood private planners have their hand forced to eliminate thru-traffic and minimize connections to arterials.

The county would basically have to do the opposite to change things; provide low-cost/low-overhead process for connecting to public road and pay neighborhoods/HOA for connecting to arterials to offload the traffick and provide thru-routes. Otherwise the public is just leaching off the private roads, and due to neighborhood planning requirements they usually can't charge a toll to get it back, so it gets designed to avoid that.

I'm suggesting not limiting foot and bike traffic just because we choose to limit car traffic. There are lots of routes between places in my town that would be much more direct, and safe, on a bike if there were small connecting paths between neighborhoods, including those built at different times by different developers, instead of being forced out onto the arterials.