I mean...doesn't this apply to anything? Why is Udemy better than YouTube? Because you get well built courses with reviews to let you know if it's good, and a curriculum. Why do cooking classes when you have YouTube? Why go to college to learn how to be a joiner (talking UK college here, not US), when you can watch YouTube?
How is Fitness+ better than YouTube? It integrates into the Apple Watch/Fitness ecosystem, which gives you all your stats in relation to the video. The video production quality is guaranteed (which it isn't on YouTube), and it's easily searchable, knowing you won't get ads for products in your search results. You can filter it by your equipment level too, so you know you won't get videos telling you to use equipment X when you don't own it.
You can do fitness stuff on YouTube if you want - just like you can do anything with YouTube videos. But if you want an easier, more integrated experience, then you sign up to specialised services. It's up to you to find where the balance of price vs quality/convince falls for you.