alt Hacker NewsLMSes have to balance a lot of directly competing needs.
It has to be simple enough for the average person to use (both on the learner side and the instruction side) and have enough complexity to allow for a lot of flexibility in setup because every organization is slightly different. They have to support 50 million file formats and everything has to be backwards compatible until the end of time and everything has to load properly and quickly on 50 million different device/OS/browser combinations. Yes, there's SCORM as a standard, but even that is rickety, and an LMS that doesn't support non SCORM files is dead in the water anyway.
They're simple(ish) in code, and a nightmare in requirements.
Everything you say is true, and yet it's clear you've never used Canvas.
Canvas is decidedly, not fast, fails to display even trivial files (such as source code) as well as more complex files that should just be handled by the browser (such as video), and it has a non-intuitive, verbose, and tiresome interface that would have felt old-fashioned 20 years ago.