I've played both, and Witcher 3 definitely sets a higher bar in my opinion. When it comes to content it eclipses botw.
BOTW has quite a basic world. It's really not very varied when it comes to content. If you actually think about it there's not an awful lot of unique content in the game, shrines are numerous and very similair, random events you can encounter are very sparse (oooh look the 8th person I've talked to that is an assassin hooray). There's very few types of enemies in the game. The Korok Trials were a highlight because they were finally something a bit different. Even the inside of the divine beasts all felt so similar.
Witcher 3 has 100+ hours of content with fully voice acted side quests and stories, Zelda's side quests are pretty much "Yo I want 55 rushrooms boi."
I think BOTW has a lot of things that are overlooked because it's a new direction for Zelda, but considering how long the game was in development, outside of the main questline the world is very sparse and the content very repetitive.
Every stable looks the same... that's another example... It's just copy and pasting shrines and stables and enemy camps, and putting in side fetch quests so the vast empty world isn't a completely vast empty world.
Well if you look at any game the way you just did with BOTW than of course you are going to dislike it compared to TW3. For starters BOTW is easily 100+ hours. Also graphics and visuals isn't everything. Zelda isn't trying to mirror TW3 or other openworld games, its doing its own thing. BOTW has one of the most immersive worlds iv ever played. The things you can do in it on top of the survival aspects are some of the best iv seen. The world feels alive and Nintendo did it on Hardware far inferior to any current console.. the WiiU.
But it is your opinion and fair enough, TW3 is a juggernaut of a game however so is BOTW.