Other systems (Windows and I think Solaris, but some more obscure systems likely do this as well) can do asynchronous disk IO, but it's not perfect, either.

I think the main reason Linux doesn't really do it is that asynchronous IO is an intrusive change and there would be just too much work to implement it in all the file systems etc.; i.e. "not worth it".

Many applications which are not OK with synchronous disk I/O seem to find thread pools good enough: reasonably easy to implement, reasonably portable, usually performs ok.