Torches are not really optional if you're doing any caving, as the twisty tunnels are super dark. Torches not stacking means that if you want to be prepared you have to utilize a sizable portion of your inventory to keeping a light source available.

I get that having torches of different durability stacking would be a hassle, but the slime torches that are unused all stack together, can we get the same for regular torches?

Crafted glow mold torches don't stack either; as soon as it has a durability it takes up its own slot.

But think about it from a different angle: instead of taking 5 torches into a cave, just bring a stack of Sap, Sprigs and Dry Grass. It takes 3 slots, but lets you craft at least 10 torches (unless you bring more dry grass). Plus, it also lets you make a roasting spit for food (since pebblets are basically found everywhere, but sprigs and dry grass are not always nearby)

If you want to keep more inventory slots, just carry glow mold stalks and quick-craft them into glow torches as needed. The quick-craft wheel is easy to forget about, but exists for exactly this purpose.

Then of course you upgrade to an ever char torch, and only need coal or glue to repair it indefinitely

Agree, there is a torch option, which makes you plan slightly ahead with materials, but the glow mold stacks are in every cave. You can never be left without a glowing option inside a cave. And after all, once you get the ever char torch you don't have to worry about light again. It has high durability and on top of that, once you unlock glue ... it practically eliminates your torch worries. I was annoyed at first, but gives you a sense of progression.