Because so few understand it and so many things use it.
If you read a guide on setting up a website. They might have you change a DNS record and you might not realize it’s doing something else. Web developers frequently want to make changes to DNS and will change the name servers away to theirs but not migrate any of the records for anything besides the website. They’ll break EVERYTHING but hey, the website will load.
If you read a guide on connecting some service like hubspot. They will have you add spf records. But those need to be included with the existing ones and not just replacing what’s already there. Mess it up and every single email you send will get sent to spam folders.