The graphics primitives the web provides are actually not that great at emulating Flash's specific graphics model. SVG and canvas differ from Flash in a few key areas, especially to do with masking; and WebGL/WebGPU are very low-level and rendering vectors on GPU is hell.

This could have been fixed by having Animate switch to an SVG+SMIL renderer for HTML5 projects, but Adobe wanted a quick fix to keep Animate relevant beyond Flash Player.