One way of thinking about diffusion is that you're learning a velocity field from unlikely to likely images in the latent space, and that field changes depending on your conditioning prompt. You start from a known starting point (a noise distribution), and then take small steps following the velocity field, eventually ending up at a stable endpoint (which corresponds to the final image). Because your starting point is a random sample from a noise distribution, if you pick a slightly different starting point (seed), you'll end up at a slightly different endpoint.

You can't jump to the endpoint because you don't know where it is - all you can compute is 'from where I am, which direction should my next step be.' This is also why the results for few-step diffusion are so poor - if you take big jumps over the velocity field you're only going in approximately the right direction, so you won't end up at a properly stable point which corresponds to a "likely" image.