Not that there isn't a lot of room for improvement, but while I can't say I've been to every major city in the US, I've never had a major issue getting around once I'm inside the city. Even if things are spread out a bit, there's sidewalks and crosswalks, which is all I personally need to consider an area walkable. And public transportation will usually get you to different areas of the city even if you may still have to walk a bit when you get off.

Admittedly I'm probably more willing to walk around than the average person, and not everyone is capable of walking that much, so like I said, still lots of room for improvement.

My biggest issue tends to be getting into the city in the first place, or getting from one city to another. From my home in the suburbs I can drive to pretty much anywhere in my nearest city in about an hour or less as long as I can avoid any major traffic jams. If I try to take public transportation though, im looking at an hour walk before I get to somewhere I can catch a bus (which only comes a handful of times a day,) and then a couple more hours before I get where I'm going, probably having to transfer to a different bus or train at least once along the way. If I drive a half hour or so to a train station then I can get right to downtown pretty easily, but the train only comes about every hour so if I don't time it right and miss the train it's significantly faster for me to just drive the rest of the way than wait for another train. Then they mostly stop coming at about 11pm, which means if I'm going into the city for a concert or something, I'm cutting it close and may not be able to get back home on public transit.

And if I'm trying to get to another city, I'm pretty much SOL. I'm basically at the halfway point between that major city and 2 smaller cities, and there is no transit options to get to those 2 other cities from where I am.