Why so few websites fix their header/nav???

Question

I am working on my own website and often take a look on different websites to see what they have to offer (Design,etc..).
I have noted that, not a single website i have looked at, fixed their header/nav. So when i scroll down the header/nav disappears.
Then i was wondering if there is a reason behind it, because i prefer having it fixed.
Thanks for your answers!!

Answer 1

Some websites likes their nav. not fixed at the top of the webpage, so you have to scroll to top to see it .
Note : If you like it fixed then then OK , make your website with your creativity .
Good luck🍀🍀

Answer 2

I also like fixed i think is better