Cycle though dropdown list using keyboard on Mac
Looks to me that this is an Adobe-specific problem. In standard macOS dropdown menus, you can navigate to and select the item you want using only the keyboard. 

Up/Down keys move the cursor on the dropdown, while space is how you select the item you want. You can navigate to/from the menu using Tab. Here's an example.

Bonus tip: you can also quickly navigate the selected menu by typing out what you want to select, for example in a select a state menu, you can click it, then type T-E-X to select Texas.