That says more about your lack of both knowledge and imagination than the vendors.

Intel added SMM for software backward compatibility so DOS didn't have to learn how to suspend and resume on early laptops. Instead of waiting for operating systems to grow support for power management the SMM firmware took over when the user closed the lid. The operating systems of the time didn't know or care and user were happy it worked out of the box. Only later when hardware and software became even more complicated did it stop working reliably.

It was also used to emulate devices in firmware on cheap low-end systems so they looked like well known hardware to existing software.

You can claim it's an evil hack and I would agree, but it wasn't malicious. Also I think you can reasonable assume nobody at the time expected that this hack would last so long or that PC compatible laptops would ever have multiple CPUs.