uml - Understanding why the extends arrow points in the opposite direction - Stack Overflow

In class diagrams I typically see something like ClassA extends ClassB where the arrow head points to ClassA. Example, here. This has always confused me. Why is the arrow head not pointing to ClassB?

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uml - Understanding why the extends arrow points in the opposite direction - Stack Overflow

UML class diagram arrow types, Gleek

oop - Explanation of the UML arrows - Stack Overflow