Speaking from experience, it's primarily due to 'tradition.' All of your combi-nozzles and rate control valves are rated and marked in GPM (Gallons per minute) and hose is purchased and stored as 50' sections. NFPA specifications for the hose required on a first due engine are in feet.
A department can't decide to use metric alone. The entire industry first would need to support at the minimum both standards.
Many of those deaths will have happened regardless of the response.
In some sense, across 300 million people, even 3000 is close enough to zero (there's more than 2.5 million total deaths each year in the US). That doesn't mean that improving fire response isn't important, but it might not be a very good place to look for incremental improvements in mortality.
I agree. I've never been on a fireground and had someone yell, "We lost the house because you didn't divide by 12!" Fireground math is a very rough science. Every engine in our district has pre calculated charts for every cross lay and discharge so you aren't doing the math on scene. Additionally, once you deploy line you have a radio and can ask the engineer to increase or decrease pressure with no math required.
A much bigger issue is the lack of recruitment and volunteerism in the American fire service. People simultaneously don't want to increase property taxes to fund career departments and they also don't want to volunteer. Communities can't have it both ways. Lack of staffing is a far greater risk than whether I'm dividing by 10 or 12.
I agree with @maxerickson, I'd bet that that number is approximately 0. The reasons people perish from fires rarely if ever have anything to do with a math mistake on something like this.
If they were taught in a US school for the last 40 years they were taught metric. If they were enlisted at any point in the last 30 years they were taught, and used, metric. If they can't handle the metric system then they probably aren't that good at math and are just using rote memorizations.
"How many pints are in a 5-gallon pail? How many cups are in a 5-gallon pail?"