There is truth to this perspective but it's also missing one of the fundamental purposes of writing essays in an educational setting. Writing essays isn't just about evaluation, it's also about teaching you how to think.
The process of reading textual material, thinking about it, and then producing more textual material about the stuff you just read (and maybe connecting it to other stuff that you've read in the past) is a critical way of developing thinking skills and refining your ability to communicate to an audience.
The value of that shouldn't be overlooked just like the value of basic numeracy shouldn't be overlooked because we all carry calculators.
You're right that it would be better if post secondary institutions would test people's ability to think in more ways than just what they can regurgitate onto a piece of paper, if only because that can be easily cheated but that doesn't mean that there isn't personal benefit in the experience of writing an essay.
I may not be the best writer but I am a better writer because I wrote essays in university, and I may not be great at math but I can reason and estimate about a variety of things because I have taken many math courses. These things have ultimately made me a better thinker and I am grateful to have had that imparted to me.