The short answer is: Because that's how the site works.

Here is a [list of privileges](https://askubuntu.com/help/privileges) you can earn, and upvotes are earned at 15 reputation.

The idea here is that you are required to invest some effort into the site to get certain privileges - for instance by asking, answering or suggesting edits.

For general questions and answers about how the entire Stack Exchange model works, [Meta Stack Exchange](https://meta.stackexchange.com/) would be the place to visit.