Swift code suspension point in asynchronous & synchronous code [closed]

I keep reading that synchronous function code runs to completion without interruption but asynchronous functions can define potential suspension points via async/await. But my question is why can't synchronous function code be interrupted at any point, can't the OS scheduler suspend the thread/process at any point and give CPU to a higher priority thread/process (just like it happens in any OS)? What am I understanding wrong here?