If you flip the perspective and look at it from an async-first point of view, then OS-level processes are just a task implementation where every operation is a (potential) yield point and the state passed between them is the C stack. There are two flaws to this, one arguable, one less so:

1. Yield points are implicit rather than explicit, so their interaction with other effects is unpredictable. https://glyph.twistedmatrix.com/2014/02/unyielding.html has a description of the problem; more generally think about the reputation that thread safety bugs have. (There are those who argue that the advantages of implicit pervasive yielding outweigh the disadvantages).

2. At every yield the processor has to swap out the full C stack (usually 4K or 8K). This is a slow operation ("context switch") and inefficient when often the only information that actually needed to be passed from one task to the next was a single integer (e.g. a socket ID, user ID, SQL query ID, etc.) or something similarly small. Whereas with userspace scheduling, a task switch only has to pass the actual task state that's needed for the task in question.