5.) Contrary to popular belief, D&D is not an easy thing to code. Sure, it looks simple on paper, but having a character generator that can effectively allow anything (in terms of either future content or worse, user-created content) yet remain stable, auto calculating, and functional is a tall order. 2e's Core Rule's 2.0 + Expansion is perhaps the best D&D software ever made, and even with the expansion allowing custom classes, lots of workarounds, limitations, and "note this on your sheet" stuff.
I think you overestimate the complexity of the D&D rules.
Sure 5Es "rulings not rules" philosophy does't translate well into a program. But eralier editions were well suited for it as they were deterministic. The Temple of Elemental Evil PC game in the 3E area was very close to the PnP rules, so it is certainly possible to code it. It just that WotC/Hasbro doesn't want to spend enough money to hire people who can do it and are also averse to other forms of contend creation and financing (fan creations or Kickstarter) and are actively blocking them.
Compare that for example to Shadowrun. Sure, someone will likely soon come in and throw around some degratory terms about how SR is so unimportant that they do not need "to protect their IP" like Hasbro does, but Shadowrun has several fan created online tools which work very well (for example the character creator Chummer) which is xml based and very modular.
And everyone who plays Shadowrun would agree that this rules system is a lot more complex than D&D.