There's a gap between accepting that the mind is ultimately made up of neurons firing and what it feels like to be a mind made up of neurons firing.
Depending on one's upbringing, they likely hold deep unexamined beliefs - bedrocks of thinking - that more or less boil down to: the mind feels like some mystical thing. People can believe in the science of biology and conceptually understand that they're the product of long spans of evolution while still feeling like the mind is quite a mystical and mysterious thing.
And it can be hard to cut through this feeling without intentionally uprooting those unexamined beliefs. I know this from personal experience. My upbringing involved the deep conditioning of a religious world view that instills these mystical ideas. I left it behind in my late teens. It wasn't until my mid 30s that I started to deeply appreciate how embedded those feelings were, and even though I didn't believe them when examining them rationally, I realized they were still there in my underlying thought processes, influencing my interpretation of everyday things.
So I wouldn't say it's just that people "still believe the mind is some mystical thing" (although this group does exist), but that "the default mode of experience for many people is the one that feels mystical".