In aviation, weight is tightly linked to efficiency. The more fuel you take, the more weight you add and the less money you earn. On the other hand, you want to pack more to deal with unexpected headwinds or busy airports.
Following the 2008 economic crisis, banks were legally forced to increase their cash reserve to be able to withstand more uncertainty.
I understand that you get a priority landing spot if you're low on fuel but it only works when it's a marginal event. With fuel being one of the major cost of airlines, the pressure put by low cost operators, a will to reduce CO2 emissions, the advent of statistical analysis, it would be logical to see this reserve shrink with time until an accident happens; unless there is a legal limit no operator would dare going under?
I have no clue how well internationally-coordinated airlines regulation are (I would say not so good as the 737-max re-certification is done separately at least in the US, Europe and China), so any country specific value or international guideline is appreciated.
