Asynchronous code is not about syntactic sugar. It's about the capabilities of having code run and also continue to have other code run.
Otherwise your code becomes synchronous which means your code blocks other code. For back end this is not desirable because you want your code to be able to handle multiple operations.
And yes Flask is used, Dajango as well, there is .Net with C#, there is Ruby on Rails, Spring with Java, other scala alternatives, and my personal favorite Phoenix with Elixir.
But like other answers you really need to specify what you are doing to get a better answer. All these frameworks are better suited for certain applications. Heads up though Phoenix was created to replace Rails. Even Elixir was designed for easy transition for Ruby coders.