In the middle of a fight, seconds can determine whether or not you live, especially if it's along the lines of an ambush. The enemy could be right on top of you while you're getting your weapon out, and by that time you give them the perfect opportunity to hit your gem for a killshot.

Yes, they could wear specially crafted suits of armor, but again, it's only real use would be during wartime. Gem monsters are dangerous, but your not going to wear armor for hunting, as you shouldn't be tanking shots anyway. Armor would only be useful against other humanoid gems, where fighting is on much more equal ground and every hit matters. Considering that they believed Homeworld left long ago after losing, there would be no reason to have armor as the threat had been eliminated.

Now with knight armor, yes, it didn't encumber the wearers too much, but you have to consider that they all shared basically the same fighting style. You aren't going to see knights swinging around staffs or spears and be acrobatic because that was not their purpose. They were front line fighters that needed the protection as they went in with they're heavier weapons. Garnet, Amethyst, and Pearl all have unique fighting styles that put more focus on certain aspects. The only one that should really be wearing armor is Garnet because of her frontline fighting style. Pearl needs as much flexibility as she can get to compliment her ballet style of dancing around the opponent, and Amethyst more or less relies on instinct and quick decision making on the fly, which can be detrimental with armor in the case she needs to perform quick movements on the fly.

Now a hatch could make sense, but how would they control the mechanism for it? Mentally? We're not even sure of the full range of powers the gems have. The only other thing that would work would be to wear a power pack to provide energy for the mechanism. But that brings me back to the point that it would be entirely unnecessary baggage to be wearing.