Basic expectations are now higher. UIs are more visually complex and subtle. You can't just freehand them with a rectangle tool and a few canned textures. UIs are expected to reflow to devices with different screen sizes and dot pitches. Users expect their programs to give them up-to-the-minute information from the internet, with slick animation and nuanced typography. With flexibility comes inescapable complexity.
Could you make a tool as easy to use as Hypercard, and let modern users create useful applications for themselves? Sure, but the results wouldn't be nearly as nice as the applications built with conventional tools that live alongside them. I have a tremendous fondness for Hypercard, but I don't think you could ever make something today which was empowering and full-featured like Hypercard was for its day.