Because we don't all share an origin, destination, and schedule. Public transit can be extremely effective for the parts of the journey that it covers, but the extreme ineffectiveness of walking the other parts can cancel all that out. You have to completely rethink density and development patterns to get to a point where most origins and destinations are near stations. That would take decades even if we wanted to do it, and SB827 was just defeated in California. Then you have to run trains frequently enough that schedules don't matter. That's expensive as hell and the necessary capital projects take decades. And even if you do them, they might deteriorate and regress later if you don't keep up with the maintenance (see: New York).
When I lived in Berkeley, BART covered 90% of my commute distance, station to station, in 25 minutes. My actual end-to-end commute time was over an hour, basically a wash compared to taking an Uber over the Bay Bridge at rush hour. Bicycling to my local station got it down to maybe 50.
25 minutes is an entire commute the average American. Being just 1.5 miles away from a station doubled that. Public transit has a wickedly hard problem to solve - even 90% coverage is still next to useless.