Try to solve an aerospace problem with this "simple" approach. No, you don't get to add strength willy-nilly, or just swap materials on a hunch. You're working with safety factors which sometimes are less than 1 (come back for references), and those physical properties of materials are notoriously stubborn to changes. You know, some Richard Feynman once quipped "nature can't be fooled, dammit" - that extends to "water boils at 273.16K full stop (standard conditions), no, you're not going to change that".

Yes, you can often do more complex design. Sometimes though it costs you centuries to choose a good combination of existing physical properties (architecture), and sometimes it's "just" decades (Gravity Probe B). Wanna propose similar approach to software? :)