When your shields are wiped, you still have your healthbar.
When your healthbar is wiped, you die.
Of COURSE health is going to take longer to replenish because it's more fragile.
The shield and healthpacks are there to supplement your fights and to replenish yourself after squad wipes. It's made that way so you cherish your health more and play less run-and-gunny, especially after someone knocks your shields off. It gives you a feeling and sense of urgency. Use cover, keep distance. Don't "thirst".
Healing in the middle of battle is supposed to be risky.
Healing while your squad is fighting after you're revived is another drastic decision you must make. Do you trust your aim and tactics enough to support your squad while they engage or will you cower behind cover and save yourself?
The game is about making split-second decisions that either bring you closer to glory or damning yourself to hell. How is this not apparent?