It's not. Democracy has problems though. The Bill Of Rights is undemocratic. It says that there are some things you can't do even if you can get 50% +1 of the population to agree to it. Federalism is undemocratic when the States interact with the Federal government but the upside is that it's harder a majority to run roughshod over the minority.

Democracy is rule by the people. That is not the same as majority rule. Consensus-based decision making (including wiser approaches like consensus-minus-1) requires supermajorities to make decisions, but that does not make them undemocratic...

Likewise the amended constitution limits the ability of the Congress to pass certain laws, without amending the constitution first.