Truth be told, I can give a flying [censored] about the lore now specially around Vampires it has still and always will be the stupidiest and not thought out thing in the game and the way they deal with it? Oh lets put in a thing that we will put into the lore dealing with this guy maken a pack with so and so here ya go. And that is suppoused to make it all dandy?? -___o
Lore > Game.
Especially in TES.
If you play the games without giving any attention to the lore, well its certainly possible and its a choice people can make, but it does mean that like with an iceberg youre barely scratching the surface.
Vampirism came to being because Molag Bal wanted to mess with Arkay. He [censored] a woman who then died and became the first vampire.
Some vampires have entered into a pact with another Prince, Clavicus Vile, who is the Daedra most close to our traditional devil, in that they both like to collect souls in exchange for deals that never turn out exactly like the wisher intended.
As has also been pointed out, the mechanism makes perfect sense.
Because blood works to invigorate a vampire with the life of its victim, thereby bringing it closer to a semblance of life.
It is also not uncommon at all in other works of fiction for vampires to require regular feeding to stall turning into an abomination, I am for instance reminded of the works of Anne Rice.
That you dont like it, fine. But dont go pretending it 'makes no sense' or is 'stupid' just because of that, when it makes demonstrative sense from a lore point of view, a mechanical point of view and is a common way to describe vampirism in other media.