Walkability tends to correlate very much with population density. If your neighborhood has big yards, ample parking, open space, these are things you need to walk past on the way to your destination. That takes time and limits the number of destinations in walking distance!
My parents live in a suburb of Minneapolis. Their neighborhood is full of green grass and trees. Their house is pretty typical for the area: a 42-year-old detached single-family residence on a one-third-acre lot. The neighborhood borders a giant forested park with hiking and biking trails. The trails are nice! I enjoy using them when I visit. However within a mile walking radius of the house there's nothing at all except that park and other houses. A little over a mile away is a car-oriented strip mall complex with some basic businesses to support the neighborhood: a gas station, convenience store, hardware store, hair salon, daycare, bank, gym, couple of restaurants, etc. The parking lots for these businesses use twice as much land as the actual buildings. The nearest grocery store is about two miles away.
Compare that to my neighborhood in Seattle. The neighborhood was first developed when the Model T was in production, and largely predates the idea of the automobile as the primary mode of local transport. My house sits on a one-tenth-acre lot. Most of the homes in the immediate vicinity are similar, though even denser development is common in nearby neighborhoods. The result of this higher population density is a much wider variety of destinations within walking distance. Within a mile walk of my house there are five supermarkets, three elementary schools, a high school, two public libraries, a subway station, a couple dozen restaurants, a food bank, a community garden, four parks, and a bunch of other stuff. Two of the parks feature forested walking trails. Much less of the land is devoted to surface parking. This is the sort of environment you need for walkability.
Bikeability is a bit of a different story. People tend to bike about 4-5x faster than they walk, so having things spread out a bit more isn't as much of an impediment. What matters more there is whether the infrastructure is set up to support biking safely, or if everything is designed only for people to zip between places at 45 MPH in their metal boxes, and bikes are seen mostly as an obstacle for drivers to become enraged at while they're waiting to pass.
My parents' suburb isn't actually that bad for biking. The neighborhood streets are pretty safe and pleasant to bike on directly. The larger arterial roads are lined with foot/bike paths. Not exactly pleasant to ride on these next to four lanes of fast car traffic, but they're safe and efficient enough. Although the nearest public library is two miles away, it's only 10-15 minutes by bike which isn't bad at all. Similar story with nearby schools and other destinations. Biking is far from the dominant (or even very common) method of local transport there, but it's feasible enough if you want to do it.
