I’m pretty sure one of the reasons is because inheritance implies method
inheritance, which implies being able to override methods, and that opens a big
can of worms which will be familiar to any experienced C++ programmers.
Swift struct methods aren’t dynamically dispatched; they’re just function
calls. But if you could subclass a struct and override methods, then the
methods would need to be dynamically dispatched … but that implies that structs
would need to contain vtables (or isa pointers), which makes them a lot more
heavyweight. Or on the other hand, if struct methods stayed statically
dispatched, then overriding them would be fraught with peril, for the same
reason that overriding nonvirtual methods in C++ is generally a very bad idea.
—Jens
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