Lets not forget, arguably the most important reason.

Because it makes us feel good.

We can feel superior to others, because we can do something that few other people can. We can collect instances where our approach is beneficial and use that to validate our self worth. And we can form a community that validates our strengths and ignores our weaknesses. All perfectly reasonable motivations (provided our satisfaction is a reasonable goal).

In my own field (Computer Vision), there are those who pursue it rationally (with rigorous mathematical analysis) and those who pursue it heuristically (creating a variety of systems and testing them on small samples). These approaches seem to mirror the determined search for truth and the pragmatic "go with what feels like it works" approaches. Without rigorously analysing them (although this may be possible) both approaches seem to deliver benefit with no clear winner in terms of delivering approaches that are practically applied or used as the basis for further work. I think it is interesting to apply this meta analysis to reason, i.e. can we scientifically determine whether approaching problems reasonably conveys advantage? Is there an optimal balance?