whenever I see the word 'reductionist', I wonder why it's being used to disparage.

a materialist isn't saying that only material exists: no materialist denies that interesting stuff (behaviors, properties) emerges from material. in fact, "material" is a bit dated, since "stuff-type material" is an emergent property of quantum fields.

why is experience not just the behavior of a neural computer which has certain capabilities (such as remembering its history/identity, some amount of introspection, and of course embodiment and perception)? non-computer-programming philosophers may think there's something hard there, but they only way they can express it boils down to "I think my experience is special".

Because consciousness itself cannot be explained except through experience ie consciousness (ie first person experience) - not through material phenomena

It’s like explaining music vs hearing music

We can explain music intellectually and physically and mathematically

But hearing it in our awareness is a categorically different activity and it’s experience that has no direct correlation to the physical correlates of its being

The common thought experiment is the color blind researcher experiencing color for the first time(Mary the Colour Scientist https://en.wikipedia.org/wiki/Knowledge_argument)