I agree, that's the main barrier. A subway line might cost a few billion, but reorganizing a major urban area to be walkable takes decades.

Part of it is that most American cities are very new in comparison to their European and Asian counterparts.

London, Paris, Barcelona, Tokyo, Kyoto, Seoul, etc. all have Ancient, Roman and/or Medieval cores which by default are designed for walk-ability. The modern cities are by and large extensions of those cores. The only American cities that have similar levels of walk-ability are ones that were founded and largely settled before the car like New York, Philadelphia, D.C. -- basically mid and north atlantic cities. Otherwise you simply can't walk by design.

Fixing that requires essentially rebuilding an entire city, which really isn't going to happen anytime soon. Imagine rebuilding something like say, L.A., at ~500 sq miles of broken sprawling urbanity, to be mass-transit ready.