Well Aseprite is only for the graphics when I do my pixel art, the actual engine itself is made by me, and it depends on very little. The reason for going with my own engine is that it's a lot of fun to make things yourself, and I want to program in C++ so Unity and the likes have never really interested me. My current engine is somewhat similar to Unity, but made for 2D pixel perfect graphics and allows you to do all your work in C++, even scripts, which is awesome.