grammar

Your understanding of hyphenation with adjective phrases is correct, but everyday is a word.

everyday (adjective)

: encountered or used routinely or typically : ordinary

// everyday clothes

https://www.merriam-webster.com/dictionary/everyday

So then, why is everyday written as one word, while other phrasal adjectives are written with hyphens? Just because it's so old and so common. The use of everyday seems to go back to the 17th century (Etymology Online Dictionary).

Note that people often incorrectly write "everyday" when they mean "every day." For example (all taken from Stack Overflow posts): "I would like to know if Customer Ben received unique Item 1 everyday", "I need the system to run the following code everyday", "I want to run a Job Service everyday daily at 6:00 AM."

My favorite, which I read on an advertisement for a hostel in Brazil: "Everyday Monkeys" (I was looking forward to exotic monkeys, not your everyday monkeys).