Precisely because pure democracy is just another form of tyranny.
It's been much more often the case that tyranny has come from tyrants!
By Aristotle's definition, a tyranny is a entity that subsumes all the power in the state unto itself and exercises such power in its own interests.
A parliament that possesses power over the executive and the judiciary is, in effect, a legislature and an executive and a judiciary in one.
Tyranny occurs when there is only one entity with the power to simultaneously make, enforce and interpret laws.
And whenever representative bodies are the most powerful entity in the state they become tyrannies quite quickly - precisely because they can claim popular support as elected representatives.
Debate fails when you don't respond to the words on the table.