If you're 1) rushed (have places to be / people to see / work to do), 2) the weather's poor, 3) you're female (the area's improved but it's still not the best side of town), 4) you're risking a 60+ minute wait if you miss the train, etc., it's not particularly pleasant.
Walking a similar distance along a different or safer route, where I've got the time to wander, or where transit service is such that I've only a few minutes wait (or as is more likely the case on much of Muni in SF: you've simply no idea and the wait may be anywhere from 10 seconds to hours), the situation's not so bad.
But miss your CalTrain connection and you're 30-60 miles from home in a part of town with not much to offer. Though that too is getting somewhat better with the ballpark and revitalization going on.
Like another poster mentioned walking for 20 minutes in high heat and high humidity (like 100 deg F - 38 deg C - and 80% humidity, not uncommon at all in the South) can be a pretty unpleasant and sweaty affair, especially in dress clothes.
Some people are limited mobility as well (like the elderly), and/or especially vulnerable to heat and inclement weather.
It's a little offensive to generalize about "Americans" as a whole, especially based on the comments of exactly 1 person, so please don't. 300+ million people live here, with often very different customs and practices, of different social classes, ethnicities, beliefs and worldviews, and living in many different geographies. It's not as diverse as all of Europe, but not even close to homogenous and easily generalized either.
Luckily not everyone. I've never used public transport in my suburban sprawl city in upstate NY, but luckily the roads are of moderate size, so I can do crazy (for America) things like walk/bike/run the nine miles to work, or even walk to the airport (seven miles, did it last week, only needed to bring a backpack so it wasn't too bad). I'd move to Portland for the bike-friendliness, but there's no biotech scene there yet (anyone want to help start one?).
If you're 1) rushed (have places to be / people to see / work to do), 2) the weather's poor, 3) you're female (the area's improved but it's still not the best side of town), 4) you're risking a 60+ minute wait if you miss the train, etc., it's not particularly pleasant.
Walking a similar distance along a different or safer route, where I've got the time to wander, or where transit service is such that I've only a few minutes wait (or as is more likely the case on much of Muni in SF: you've simply no idea and the wait may be anywhere from 10 seconds to hours), the situation's not so bad.
But miss your CalTrain connection and you're 30-60 miles from home in a part of town with not much to offer. Though that too is getting somewhat better with the ballpark and revitalization going on.