Why is there no PDF downloads or mobis and everything is mostly zip or epub

Because epub is the format in which ebooks are widely available.
It also happens to be very easy to convert into other ebook formats and also supported by all generally available (non-proprietary) ebook readers.
Since PDF is just an image with fixed fonts, size and non-flexibility, it is not preferred by those who read for fun.

Most devices (e-readers specifically) support epub so that's mainly what people want and use. PDF's and the like are usually on computers, but you can download programs that allow you to read epubs on your computer if that's the issue.