Because they are search engines and not directories. Because they allude to the searchers that they can provide the relevant information they are searching for
Anyone (apart from ppc arbitragers) who spends money to be found for a service they don't provide is not going to be around for very long. Are you trying to say bigger companies are less relevant to the searches they target i.e. they are less able to provide the services people are looking for? I don't think so. In fact I think precisely the opposite. They are successful because they are good at what they do.
which we allow to fill their indexes for free while crawling our hard work (our work, our bandwidth=money
Well block them then. Or do you actually WANT the traffic? Well then do what you have to to get it!