Using a Looting III sword on an enderman used to almost guarantee an ender pearl drop, often more than one. I hunt down every enderman I see in the overworld, in a few different realms, some without Looting III swords (new realms) and some with Looting III swords (established realms) and ender pearls are just as scarce in one as in the other.

In fact, in my established realm, my count of killed endermen without a single pearl drops using a Looting III diamond sword (Sharpness V, Unbreaking III, Mending, if that matters) currently stands at 12. A full dozen endermen killed without seeing a single pearl.

I don't know if this makes a difference, but it, if memory serves, it seems these endermen were all carrying blocks of dirt or grass, and the only drop I got from them is the block they had been carrying. I haven't noticed if I get pearls from endermen NOT carrying blocks yet, but I'll update this if I do.

I revisited a realm I had been playing in last year and I had almost a full shulker box of stacks of ender pearls. In my realm I started this year, on which I have played maybe half the time I spent on that one so far, my surplus ender pearl count stands at three-and-a-half stacks, and I think I've killed a lot more endermen here because I know how to kill them a lot better than I did then.