It really sucks that you can't increase health or stamina by upgrading Fortitude and Agility. I can no longer play and a tanky or speedy character because I can't upgrade my health or stamina anymore.
leveling up does that (add stamina/HP) and by speccing into stats makes you more efficient in those regards (heavy armor, or even light) and (pain tolerance )for tanky-ness and cardio fixes sprinting stamina usage
Health and Stamina are the LBD components of the game. Just keeping surviving and those attributes will passively increase over time. Smell the A16 fumes.