Why? Go use the search button of the forum and you can see that there are too many threads asking the same questions as yours. There's no such proper opinion related to these question(s).
People believe in something coz they're into it and mostly they invested money, time, so on and so fort.
I agree that this kind of question has been asked several times in this forum with a rephrases words and people who answered it have a different perspectives and opinions to this matter. Honestly, we cannot really point out why people or most of the people in here believe to bitcoin but I assure that they believe or rather we believe in bitcoin because we know what goods it has brought to us and what benefits we gain from it.