Why hasn't the option to hide certain questions (that are for example on the first page of a Stack Exchange site you regularly frequent) implemented yet?
This option is already implemented. You can add ignored tags, and decide if questions with the tags you add there are hidden or just greyed out.
For the browser it is available at the right sidebar:
Hiding specific questions wouldn't be really worth it (at least not for higher traffic sites). These questions would slip away from the top quite quickly.