You aren't conscious when you're in a coma, correct? That's a measurable way the system can mess up and we can detect. You also aren't conscious when you're dead, right? Yet another measurable thing. We can detect brain activity and see certain regions are used for certain things. We can also detect anomalous behavior in the brain. We can tell when the system isn't working as expected.

Nobody has anywhere near a coherent account of how a purely physical system produces (or equates to) subjective conscious experience.

We can easily explain how a physical system produces consciousness. We may not be able to point to exactly what it is, but we can describe it and describe how that can happen. It's not mystical. It's just complex. We can't reproduce it yet, but that doesn't mean we don't understand how the brain functions.

Why should science be forever married to a reductive physicalist account of the universe?

Because that's literally a basic requirment of science. It relies on falsafiability. You can believe whatever you want, but science relies on stuff being measurable. It doesn't mean it's right, but that's how it functions.

Also, you call it reductive. I don't think it's reductive. I think it's more reductive to just say "consciousness exists" than to say "consciousness is a complex system that can develop in nature". Just because it's physical doesn't mean it's reductive. Saying "it just is because it is" seems much more reductive.

Edit: Also, despite people believing mystical things for most of history, they were never right. Why should this be any different?