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.

I've played both games and tbh I wouldn't really compare them, I like them both individually. Helldivers 2 is a lot more focused on combat and fighting overwhelming hoards of enemies. On higher difficulties sometimes stealth or just running away is the best solution because of how absurd the number of enemies are. Its honestly really fun just trying (sometimes in vain) to thin out enemy numbers while protecting an objective. That being said, everyone gets access to cool down based, very powerful orbital abilities that do everything from calling down powerful support weapons to dropping a 500kg bomb on a bunch of enemies. At the moment there are 2 different enemy factions that require pretty different playstyles to overcome as well. Honestly pretty dope game, check out SkillUp on YouTube for a good review of the game.

I have played both DRG and HD2. I think you simply have to play HD2 to answer that question. It might not be for you, but having played both games "a lot flatter and less tense" is how I feel about DRG.