If I want multiple animations, I just draw animation 1 and then draw animation 5 for example.
The kind of animation you’re talking about is essentially analogous to sprite animation in 2D. You have a series of bitmaps and you pick which one to draw based on the animation. So if you have a 3-step walk cycle, you draw frame A, then frame B, then frame C.
However, you cannot create a frame between A and B without drawing another frame. You can’t create a frame halfway between A and C algorithmically; you have to draw it beforehand.
With skeletal animation, everything is done with a series of matrices. So you can combine multiple animation frames to get the final correct animation.
I also don’t know how to calculate the vertex coordinates for bone animations
Maybe you should learn. Vertex animation is sometimes used these days, but it is quite rare. Skeletal animation just has too many advantages to allow vertex animation to be used in anything except a few specialized niches.