Scientists will chase chemical soups as long as it is the most probable theory, it is that simple. The thing with chemical soups is that some of them form very complex structures, so it is pretty easy to imagine that the right kinds of soups was the origin of life. Everything else people have suggested are either much less probable (not sure how it could happen without chemical soup) or they are nonsense (intelligent design, ie "life was created by life that was already there, just don't ask me how that other life appeared").

I don't see how one could rule it out, since we don't know the number of planets on which the conditions required for abiogenesis could have occurred. It's not limited to the # of planets in the volume of the universe observable from Earth.

Right, given the Anthropic principle the probability has to be nontrivial when multiplied by all the oceans and the lifetime of the planet, but also by billions of planets and billions of galaxies. So there may be no way of ruling it out by any experiment.

All you can do is build bigger vats, just like physicists build bigger supercolliders, hoping for a result.