Yet your character doesn't ever use the bathroom. Your character doesn't dream when they sleep. You have to assume these things happen even if you don't see them, so when you fast travel, why can't you do the same? Why can't you say "My character walked all the way there" instead of actually having to walk yourself?
Because spending several hours (of in-game time) traversing the open countryside is not the same thing as sitting on a bucket for a few minutes taking a dump, or going to bed for a few hours in a relatively safe place (an inn or your own home).
In the latter two situations (with rare exceptions) it is highly unlikely that anything potentially interesting is going to occur while you are doing those things. You are not likely to have a random encounter with a potential friend or foe while tucked away safely in bed somewhere, and you are probably not going to discover any previously unknown locations while dropping the kids off at the pool.
However when you are out walking from place to place, stuff HAPPENS. It may not even be stuff that you take part in, just stuff that you witness. I've been playing for almost 8 months - I can't even tell you how many hours, probably averages out to 4-5 per day with a few weeks off here and there - and I am still seeing things happen that I never saw before. I would never see them at all, or see a lot less of them, if I made a habit of fast traveling.