Why do threads share the heap space?

Question

Threads each have their own stack, but they share a common heap.

Its clear to everyone that stack is for local/method variables & heap is for instance/class vari

Answer 1

Processes don't --generally-- share heap space. There are API's to permit this, but the default is that processes are separate

Threads share heap space.

That's the "practical idea" -- two ways to use memory -- shared and not shared.