Probably the same reason PoW is wasted on "useless calculations" instead of calculating protein folding or whatever.
Because these useless calculations have cryptographic properties (one way functions) that make the process secure. Useful calculations instead of just calculating a simple hash function would either be risky in term of future security (because of unknown properties), or outright insecure.
Completely off topic and not really relevant to your comment buuutttt Banana coin’s ‘POW’ is based off protein folding and contributing to medical research. It’s a joke and you aren’t enabling consensus but you are getting banana emissions for ‘noble causes’, which I found was fun and regularly run it when I go to sleep.
The process of plotting the data is strenuous on SSDs. You might kill an SSD if you are plotting a petabyte or so. But if you are that serious, you can plot in RAM (or just eat the cost, which is insignificant compared to the cost of a petabyte).
The process of farming/mining doesn't stress HDDs in the slightest.
The point is to save energy during operation in exchange for spending more energy on manufacturing the equipment. Since equipment manufacturing is bounded by more than just raw energy the end result is a net decrease in energy consumption.
And in reality much much less than a Datacenter would. Once plot files are written they use a cache to look up their hashes. The utility of those files on the other hand are a whole different conversation.
Because these useless calculations have cryptographic properties (one way functions) that make the process secure. Useful calculations instead of just calculating a simple hash function would either be risky in term of future security (because of unknown properties), or outright insecure.