After a bit more reading, it seems I hadn't realized just how low level direct system calls actually are (https://en.wikibooks.org/wiki/X86_Assembly/Interfacing

After a bit more reading, it seems I hadn't realized just how low level direct system calls actually are (https://en.wikibooks.org/wiki/X86_Assembly/Interfacing_with_Linux https://en.wikibooks.org/wiki/X86_Assembly/Interfacing_with_...). Apparently it's either a software interrupt or dedicated instruction, with the constant in a register. ASLR requires more abstraction than that (obviously), and there's no point to it if an exploit can just use the front door instead.