As somebody who has paid good money to run all across the property for the WDW Marathon in January (2013 and upcoming 2015), I don't think it would be a good use of resources for them to invest in a broader network of walkways that the vast majority of guests wouldn't use. Walkways already exist where the distance is more manageable -- Contemorary to Magic Kingdom, Boardwalk resorts to EPCOT and Hollywood Studios and, by extension, EPCOT to Hollywood Studios.
That last one is the longest at around 1.25 miles and would take roughly 20-25 minutes to walk for the average adult who isn't toting any children, strollers, diaper bags, etc. It's certainly a doable distance, but it's been a pretty lonely walk the few times we've taken it. On average you might save 15-30 minutes over the bus or friendship boats, but it's still more physical effort and exposure to Florida heat/humidity than taking either of those, and most guests don't tend to be physically active and childless.
EPCOT is closer to Magic Kingdom than Hollywood Studios at about 5 miles, give or take. Animal Kingdom is about 5 miles from Hollywood Studios (6.25 from EPCOT) and about 7 miles from Magic Kingdom. The moderate and value resorts are spaced around and between these, but most would be looking at longer walks than that of EPCOT-Hollywood Studios just to get to the closest park, let alone those farther away.
Just speaking personally, as a pair of physically active and childless adults, we would still likely use the existing transportation system in place over walking if the option to walk existed at the resorts which currently lack such options. As regular WDW vacationers, we know how much we already stand and walk inside the parks and just aren't looking to add too much more when we're vacationing. Those who are less physically active, those with disabilities and those with children would tend to have even less motivation than us to utilize such a network of walkways.