I feel like a guy yelling, “don’t drive on the sidewalk because people should be able to walk there” and your response is “nobody walks on the sidewalk because everybody drives there.”
Anyway, the larger point is: driving on the road is *better.* And I would argue using relative units is better in many cases. Why? Because the code maintenance is easier. True, using px requires less thought up front. But if you use relative units, you can define systems that work together better as a whole.
You can change a few inputs and have the entire system respond. You can define things in terms of your fonts, because your fonts are an integral part of the design. With pixels, small changes in the future can require code changes all over your code to adapt to that change. With relative units, done right, one small change can update the entire system accordingly.
CSS does a lot of work for you, if you will let it. Normal document flow ensures things all fit together on the page…keithjgrant.com
edited to add:
Taking a step back: px and em and rem are tools. An important part of CSS in understanding all of them. You should learn the pros/cons of each & choose whichever is best for a given scenario. There is no one-size-fits-all, and I think its irresponsible to tell developers they don’t need to bother with some of them.