Because it’s hard to relate to something that never occured to you before, speech is an automatic things for people, most of the times there are no weights in their word so no suprising it catch them off-guard when seeing someone stutter. Partly also because of the narrow perception created by Media, stuttering never appear as a condition, rather as a comedic relief and cheap gags. Stuttering has always been of the latter end in the disability hierarchy, because it’s much easier imagining yourself missing a limb or become intellectually disabled, but stuttering has its roots lies in the accumulating mental trauma and is virtually impossible to form insight at first glance. But the primary reason lies in the fact that, their are ONLY 5% of stutterer in total, so they don’t have that kind of encounter happening frequently enough to rise consideration.