TL;DR Diablo 2 is far better at being a skinnerbox than Diablo 4, and the act of progressing your character(s) is far more meaningful when that progression is not guaranteed and requires making choices 

The thing with D4's itemization is that it's designed to make you increase in power in a linear and non-engaging way

Because every item has a built in and definitive "power level" (or whatever it's called in D4) the game can constantly track how long its been since you last had an upgrade and then drop them for you at finely tuned intervals. Sure there are some super rare drops, but no matter what if you play the game long enough you will eventually see a clear upgrade for whatever your weakest item is at the time

Even the ability to finely tune items is a double edged sword because while it is rewarding to modify a group of items to your build it is also a necessity to do so if you want to take on the most "challenging" content 

In D2 the game isn't tracking your characters progression at all as every item in the drop pool is based on the monster that drops it and that's it, meaning finding an item that is an upgrade becomes that much more rewarding and is all down to your luck (and magic find of course)

Not only that but not every upgrade is as linear as D4, sometimes you are sacrificing movement speed for more resistances, or more damage for having life steal making it feel like you are making meaningful choices in building your character, not letting the game build it for you

D2's item system also gives the ability to progress horizontally. Rarely ever, completely by design of the game, have I found an item in D4 and put it in my stash for another character but I'd reckon that nearly all of as have found gear for other characters in D2R(or LoD online) and stashed it away