Your proposed "girl with a big round butt" test isn't reproducible. She might sit a bit differently the second time, and if two people do the test they'll probably find two different girls who weigh different amounts.
So, they standardise the test by applying a fixed amount of force in a specific way.
"Almost anything will break if it's put in a vise and continuously increasing pressure is applied to the middle." But the test isn't "can I break it". The test is to *measure* the point where the phone breaks, so they can compare that across phones, and they can compare it to the amount of force exerted when a "girl with a big round butt put the phone in her back pocket and then sit on it".