I love all three, but they are quite different in their gameplay. In DRG you choose a class upfront so your role is more defined by this choice, the challenge is mainly about getting your bearings and traversing the terrain, and the mission objectives are (IMO) more involved. In HD2, the challenge is more about surviving against hordes of enemies without killing each other. In DRG, if you shoot somebody you hear a funny voice line, but I don't think I've ever killed a teammate by shooting them. In HD2, this happens all the time.

I don’t understand why I would play it over Deep Rock Galactic, especially when the original Helldivers actually has splitscreen co-op.

I don't see the logical connection here, but you do you. Perhaps worth pointing out, the original Helldivers doesn't have splitscreen but rather shared screen coop -- meaning you can't get separated from your teammates, which is both a feature and a pretty big limitation.