Because the meaning of "Google" is clear while "Alphabet" is not.

...and the link is to a .google domain

They foster the confusion themselves.

Names stick, it’s as simple as that. In most practical situations (such as this discussion), the distinction between Google and Alphabet doesn’t matter.

I once tried to rebrand an in-house, purely dev facing product. I failed.

Same reason Facebook is still Facebook to me, probably.