Why is volume of the cancer not used?

As a layman, it seems to me that what is important about the scoring is what it tells you about the level of needed treatment.
PCa kills by metastasizing and growing in bad places, blocking the body's functions, sucking up nutrients, etc. There was a study with 20,000 patients; Gleason 3 didn't kill any of them. So Gleason 4 and above is what matters.

Perhaps the existence of multiple tumors is associated with a tendency to metastasize? OR with multiple variants of PCa, which makes it more likely that one will spread?

A larger tumor will mean it has had longer to grow (at whatever rate); not sure if that tells you anything about the tendency to metastasize (except that one wishes it had been found earlier).

"So Gleason 4 and above is what matters." Yes I am comparing volume of Gleason 4 in two examples, so definitely with you.

"A larger tumor will mean it has had longer to grow (at whatever rate); not sure if that tells you anything about the tendency to metastasize (except that one wishes it had been found earlier)."
Yes so in one case the tumor is 2.1 cc, and other it is 0.2 cc, so most likely yes the 2.1 cc tumor is older. It doesn't matter how the 3 in there changed over the years, it likely matters how the 4 changed. Since the 4 in the 2.1 cc case is way more gleason 4 it seems concerning, yet it is classified as a less serious cancer for what seems to me just something out of the past categorizations that lacked ability to measure volume even close so before MRI and so on.

I hope people are understanding what I am saying, it does involve some minor math on that Gleason 4. The volume of gleason 4 is way more volume in one but called less serious. I wonder if that should really be the case.

I'm trying to puzzle it out too, (and possibly projecting more real sense into current practice than there is; it's entirely possible that there's a strong factor of "we used to do it this way, there are billing codes, lab equipment, etc." Look at us still using the QWERTY keyboard, which was deliberately designed to slow down fast typists so the early mechanical typewriters wouldn't jam!).
I suppose an older 4 tumor might have more of a history; the doctors know more about its past behavior, such as it hasn't metastasized or mutated to 5?

Parts in tables from John’s Hopkins takes these issues into account:
https://www.hopkinsmedicine.org/brady-urology-institute/conditions-and-treatments/prostate-cancer/risk-assessment-tools/partin-tables

Partin tables (spell check)

Here is how I think they should look at Focal as I feel the criteria should differ:

1) Are there any mets/lymph nodes (need PSMA and other scans to be clear),
2) Does the ablation technique easily cover the lesions volume plus some margin (so not too big a volume for the equipment),
3) Is it easily accessible to the equipment (reachable location and no blocking things like calcifications for Tulsa/HIFU)

Perhaps more, but I don't think the same old should apply to some new techniques that aren't whole gland.