The problem is much deeper than that. Most OSes' syscall ABIs are not stable and could change without warning. What is stable is the dynamically-loaded libraries, shipped as part of the system. Linux is the notable exception here; the Linux kernel project doesn't ship a libc, and Linus is very famously opposed to "breaking userspace."

There's nothing that can stop you from using syscalls in theory, but if you want your app to be portable across different OS versions, past and future, you'd better not.

Incidentally, syscalls would also break Wine. The way Wine works is basically by shipping their own versions of Windows DLLs, which express their operations in terms of Linux APIs. Because Windows programs don't rely on syscalls, and call all system functions via the system-provided libraries, the Wine loader can just link Wine's version and let the program work normally.