Mechanical engineers went to university, and studied an enormous amount of theory: not only they studied material science, that you wrongly think is irrelevant to building bridges (it definitely is not) but they studied lots of physics, differential calculus, rational mechanics, and all sorts of theoretical subjects.
You would not want to use any bridge that was built by someone without using science. In fact the vast majority of bridges built today would not be possible at all.
Do you think Google or Apple or Oracle make software without Computer Scientists?
A lot of software is simple, including perhaps the one you made in the last 40 years, that is great, but do not assume all software is. You would not have a mobile phone or Medium or pretty much any of the software that you use daily if it was not for computer scientists.