With both ELI5 and this title, the hidden assumption which is generally correct is that "I don't remember much from my education specifically middle and highschool".
Well I do remember my highschool subjects and I feel like there is a chasm between highly detailed technical content created for other profesionals and watered down versions for 5 year olds.
I don't see that many "explain to me like I've finished highschool" content.