You can walk in SF, I also did it, but the difference is that you don't really see much of the city if you do, compared to Berlin, London, or Tokyo. In those cities you can go from pretty much anywhere in the city to anywhere in less than an hour with public transport and on foot. Less than half an hour if you restrict yourself to the touristy center area. That is just not possible in SF.
Anecdotal, but as a visitor I found it far harder to get around than, say, London. There's a public transport system, but in terms of frequency and reach it was not what you'd find in a large European city.
