I charged my Ronin maybe a month ago, and I just powered it on to discover it's at 97%. My Air 2 batteries go down to 2.5 blinking lights after only 3 or 4 days or so. Why can my Ronin hold a charge for so long but my Air 2 batteries can't? Why can't DJI make my Air 2 batteries hold a charge longer?

It's not a bug, it's a feature.
Your drone batteries self discharge to protect them from the damage caused by sitting a long time with a full charge.
And it's explained in your manual.
Look at p25 in the manual ....

Auto-Discharging Function:
To prevent swelling, the battery automatically discharges to 96% of the battery level when it is idle for one day, and automatically discharges to 60% of the battery level when it is idle for five days. It is normal to feel moderate heat being emitted from the battery during the dischargingprocess.

Labroides Posted at 8-7 17:24
It's not a bug, it's a feature.
Your drone batteries self discharge to protect them from the damage caused by sitting a long time with a full charge.
And it's explained in your manual.

Thank you for that information! Why do you think this same feature is not available on the DJI Ronin?

Dangerly Posted at 8-7 17:40
Thank you for that information! Why do you think this same feature is not available on the DJI Ronin?

Different battery chemistry. Drones use a higher power to weight battery, Li-Po (drones) vs Li-Ion for lower power draw and longer storage times (Ronin).

Hi there. Thank you for reaching out. The main reason for this is both have different battery types also the feature the was available for both units. Kindly please refer to post #2 and #5 from our fellow DJI Pilots for your reference. Please don't hesitate to reach us if you have other concerns. Thank you!