Strings are trivial in C++. Drawing 2D graphics is fine, many libraries do it. Creating old-fashioned component frameworks (like Qt, JUCE, etc) is easy. I implemented all those things on my own, decades before AI.

The hard bit is when you have complex, responsive layouts involving text. (And styling it)

Something seemingly trivial like an agent-style chat containing markdown text blocks, where it's expandable and smoothly scrollable is insanely difficult. The only frameworks that do this well are browsers, and the amount of horrible, horrible logic hidden inside them to make it look easy is hard to believe.

And since in many agents you're also going to want to display actual HTML, unless you're going to embed a browser engine in your C++ library, that's just not doable.