There's a lore reason I suppose. Vampires have never been one unified group in TES games. In Morrowind you could choose between different vampire clans to join for example.
Kind of like how if you could be a bandit in Skyrim, you probably shouldn't be surprised that other bandits would still attack you on the roads.
Haha! I remember saying that about Morrowind! Give it time, my friend. Install a mod like
Ordinator in a future playthrough to see the kind of sweeping improvements mods can make to the base game. Or even little quality of life things like a mod that makes arrows spin in the air after you fire them, like they should do. At the very least you're bound to appreciate mods like Skywind and Beyond Skyrim that are adding entirely new provinces to the game. All lore friendly, so just like the best quality DLC expansions you could hope for, all for free.