I agree that activated carbon (ac) is underutilized in the backpacking community. I think the problems are as pointed out above, the effectivity of ac cannot be guaranteed because it wears out as you use it. They don't mind for the refrigerator pitchers because you are treating treated water. They don't want to say it will remove heavy metals if you could potentially be drinking water with dangerous amounts of heavy metals.

I also think filter companies are trying to get high flow rates in a extremely light package. Additional filter steps detracts from both of those attributes. It would be very hard for a gravity filter to get a high enough flow rate and put it through a small carbon filter.

I disagree with WD that typical US backpackers wouldn't see benefits from ac. On the east coast and Midwest most open water is brown, leaf and cedar detritus, tea. It has a flavor that is definitely improved by ac. In the mountains in the west the water is typically great tasting snow melt, but the mountains have seen a lot of mining activity over the years. Heavy metals are mostly not at serious levels but they are there and long term exposure is not great for you.

As a solution, Camelback makes an inline ac filter. I have one one a bottle, but I can't seem to find them anymore. It looks like they still sell the inline attachment:

https://www.amazon.com/Camelbak-Unisex-Fresh-Reservoir-Filter/dp/B00554YH0A