Vulkan, OpenGL and DirectX make GPU programming more difficult than it has to be for multiple reasons, some of which other people here have mentioned. They are also low level, focused on high performance, and are primarily designed and written for game engines. Making them easy to learn for newcomers is pretty low on the priority list. ShaderToy makes it easy by taking care of everything except the GLSL, you might like it. You can also try CUDA, which is generally a lot easier than Vulkan. For a really easy/fun time with CUDA, try Warp - it’s CUDA in Python.