That PCs are the exception that confirms the rule, which existence can be traced back to the point where IBM legal team wasn't able to kill what Compaq has set free.
The 90's Apple, like everyone else, had its own vertical integration in software programming stack, network protocols and hardware. Also not every model had internal expansion slots, what we bought was what we got for the device's lifetime.
Naturally the more expensive LC and Quadras had enough internal bays, given their business purposes.
