Why does TQ not have a proper save?
You have to plan ahead to make sure you have cleared a zone before exiting since all the mobs respawn when you "Resume" playing.
It reminds me of a persistent world aka MMO.
I know the narrative is that monsters and beasts continue to attack, and at lvl 25 I have not resolved that issue yet, but it would be also legitimate to not have respawns once an area is cleared.

Was this a design feature so players could farm for loot and experience?
I find that if you loot and sell everything along the way you have more than enough coin and I am leveling at a steady pace by following the storyline and clearing zones just once.

Wouldn't call it a proper save, just the way TQ and some other games work. Yes, obviously it helps with farming - can't farm if there are no monsters around to fight again especially if you're looking for a particular piece of gear.

I love the game and expect I will be playing it for months, trying out various mastery builds ,Epic and Legendary difficulties;

A more accurate term would be "standard" save.

Again depends on the game itself. GD works in the same way obviously since it's built on the TQ engine. I played Two Worlds and that saves as you state - once cleared there are no respawns. Kingdoms of Amalur again doesn't - in fact that game drove me crazy because as soon as you cleared an area and walked away it repopulated and I hate that. D2 also doesn't save as you state. I don't think there is any proper/standard way of saving games; each developer decides that for themselves.