Scrolling to the bottom of the page to search for something is counter-intuitive. I'm guessing search was added a while after the header was built — but it's not like there's no room for another menu item?
Search is not free to implement and it's possible there is a 'nudge' theory aspect: its hard to find so you don't over-use it.<p>(not free as in costs CPU cycles, is not cacheable)
If you use Duckduckgo as your primary search engine, you just need to write `!hn <search terms>` in the address bar to search HN. You could also set it up manually by adding a bookmark with %s as the search term and hn as the keyword.