There's a lot of requirements creep, the bar is high, since the estabilished stacks have had decades to build up their feature sets. Complexity multipliers include accessibility, cross-platform, gpu acceleration, complexities of text in the modern world (unicode, fancy rendering like subpixel aa, fonts, bidi...), programming language bindings, memory/lifetime management in presence of different mutually incompatible PL memory management models, native code hostile mobile platforms, etc. Leave any of those out and you start halving your potential users for each feature.

(For more specialised, niche gui toolkits the answer is that they're out there, but you've never heard of them because they don't enjoy the network effects that a wider user base gets the big toolkits)