Coroutines are doable if you're programming directly in assembly, but if you want to do it in C-like structured languages, it turns out that it's really tricky: the whole concept of those languages are about having a single stack and hierarchical lexical structure. You can do coroutines in languages like this, but unless you want to do nasty things with longjmp and manually manipulating the stack pointer, they simply aren't built for it. You can build structured languages with first-class support for constructs with coroutines, but it took a couple of decades of programming language development for that to happen. Even today, most of the languages that support coroutines (C++20 included), only has support for the stackless kind. Basically the only languages with full stackful coroutine support in wide use are Lua and (sort-of) Go.