You forget that Flash was a complete, well-rounded development environment, not just a language. It's a difference between Unity and e.g. writing your own solution in C++. The later is certainly way more powerful and gives you a lot more choices and freedom, but it takes a lot of effort and testing and learning to build a really productive tech stack and workflow that way. The same thing with HTML/JS/CSS. So many frameworks to choose from, IDE's have only very basic support for them, documentation sucks (if any), no good visual tools for animation, takes a lots of steps to push data from one tool to another (exporting/importing, converting different formats, etc.), you need to install a dozen of different tools and frameworks and then you still need to code a lot of basic stuff yourself. On the other hand Tools like Unity or Flash give you all you need in a single box, all perfectly integrated to work together, documented, you can just jump right into making things.

It's the same thing that Delphi & Visual Studio did for modern programming. If you were used to working in Turbo Pascal/C and switched to Delphi, it was just amazing what a productive boost you'd experience from having a true integrated development environment with all the little tools and drag&drop builders and ready-made components.