Not everyone do that tho. And that includes pretty good senior engineers I work with.
I don't necessarily do BDUF or anything, but I generally do more "engineering" than is often directly necessary. Partially, it's my personality. But part of it is that in my experience, prototypes have a way of becoming production systems.
Last time I had to do a prototype for work, I split the difference: working quick and dirty, just meeting the minimal requirements, but also documenting what we would need to do if we decided to move forward with it. That felt like a good balance.