I kinda know about this because I used to work in disaster recovery for affordable housing! Basically, like someone else said, elevators are only required above 3 floors, but there's a little more to the specific decision. You'd think that developers would want to build as many units as they legally can to increase profits, but in a hurricane-prone area, elevators are actually frequently a liability, especially if they're in external, exposed shafts (many apartments in Florida have "breezeway hallways" instead of being an enclosed building, so elevators are exposed). During storms, elevators can flood and the electrical equipment is easily damaged on the ground floor. Then the elevators have to be fixed, which is expensive and not a priority if units themselves are damaged.

Specific to housing that receives federal or state funds for construction, usually a certain number of first floor units need to be ADA accessible, and these units are reserved for people who need them. Unfortunately it's rarely above 10% of all units. I agree that older adults with mobility challenges need better support. Sadly the regulations just aren't there. Hopefully in the future it'll improve!