Mac System 7 wasn't so much an operating system as a collection of utility functions (half of which were burned in ROM on each machine). There was no kernel, everything was completely cooperatively multitasked - software ran it's code and then called a function that would jump to the next running process.
It was designed originally to run off a 400K floppy drive within 128K of RAM. "Copying files" originally meant swapping back and forth between two floppies in the single internal drive and the system had to evacuate as much code as possible from that 128K RAM to minimize the number of floppy swaps.
It took Apple a long time to try to bring that early primitive OS to modern standards while keeping software compatibility, and eventually they gave up and bought NeXT
The Lisa, before the Mac, had a proper multitasking OS, but it cost 3 times as much as the already-expensive Mac so it had the luxury of 1 MB(!!) of RAM and a MMU. It was an utter flop.
