> If a single developer can create ST from scratch for Windows, OS X and Linux,

One of the reasons why some people are switching from ST to Atom/VSCode is the former's tortured development schedule, which has seen months or years go by without an update. Contrast VSC, which reliably delivers a heap of improvements every month. JavaScript is probably responsible for a big portion of this. Again, it's a tradeoff.

> then surely GitHub or Microsoft (for VSCode) can create a cross platform native set of UI components in C or C++,

Maybe they can. Or maybe they can't, and if they tried then they'd have the same slow pace as ST. You don't know.