The only reason anyone does anything is in the expectation that things will turn out positively for them if they do.

That's not necessarily true. People who buy lottery tickets generally don't expect to win the jack pot, but rather hope they do.

You can never know what someone else is actually thinking. This fact applies even more so to people who struggle with perceiving social cues. That's why some people struggle with forming a realistic expectation of the outcome of asking someone else. Some people, like the OP, might notice 'signals' when there isn't actually any. On the contrary, others might not notice the 'signals' that the other person is giving off. In other words, there is often a gap between what is reality, and what one expects; the larger this gap, the less useful that expectation is to decision making.

That's why, in certain situations, it is prudent to ask someone out even if you don't have an expectation of a favourable outcome. A person with poor social skills will often struggle to form a reasonable, and realistic expectation that will beneficially aid their decision making in regards to asking someone out. Should that person just never ask someone out? No. Do it and hope for the best.