There are many reasons behind it, to name a few: files are self-contained(*) and easily portable, can guarantee some security features, the format is easily extended, and the ecosystem is very large.
It seems that a better format should exist, but the fact that PDF is the de-facto for portable documents make it unlikely things can change overnight.
It seems that a better format should exist, but the fact that PDF is the de-facto for portable documents make it unlikely things can change overnight.