There's a statistic that floats around which may be apocryphal - something like 90% of visitors to national parks don't get more than a 5-15 minute walk from the parking lot (and some literally never leave the car).

National parks are huge and you can quickly literally get lost forever in them (which is an actual danger, stay on the trails!) if you're willing to walk.

Some of them have very obvious "goals" to see (the geyser, the half-dome) which of course are high traffic, but others are beautiful "all over" and taking the treks is worth it.