Same reason supermarkets place bread and milk far from the entrance, so you have to fuddle your way through the rest of their offerings before you get what you want. YouTube would have figured out if they make it really difficult to find good content, you'll get distracted looking at something else and then have to come back for whatever you wanted later anyway.

Hot tip: I use regular google search like so to search youtube 'topic site:youtube.com'. It avoids all the clutter the YouTube search results provide.

Afterthought: YouTube is getting so bad, I wonder if there's any third-party (paid or free) front end?