In areas of Chicago and Baltimore, for example, the streets are "walkable," however the frequent violent & property crimes that take place in these areas make a casual stroll down the block an unsafe proposition.
I grew up in Harlem during the time it was emerging out of the 1980s crack epidemic. As a child, the streets were easy to walk on and crime did not impact this for me in any real way. Just a few years prior, though, this was definitely not the case. And even today, as "walkability" improvements have been made in Harlem, the streets are quite obviously less walker-friendly, since walking down the street exposes you to a non-insignificant risk of being a random psychopath's next unwitting victim.
My suspicion here is that you have some sort of binary thought process surrounding cars in cities. The fact is that cities are not as livable in general. Any solution involving more or better walking capacity is a temporary fix at best. In actuality, what you need is a very solid city planner who is able to anticipate and respond to cultural trends adequately. Cities need to be able to balance the needs of various transit options, such that they don't compound peoples' other problems that come from living in cities: crime, cleanliness, affordability, etc.