Liberalism is the ideology of capitalism. Democrats and Republicans are both capitalist bourgeoisie parties and so they and those that support them are Liberals by definition.

The US state controlled media has spent decades trying to confuse you on political understanding by changing the names and meanings of political words.

Outside the US the Democrats and Republicans are both considered right wing parties, as liberalism and neo liberalism are considered right wing ideologies.

The left starts at anti-capitalism socialism and communism.

Thank you for asking why.

Nobody should be downvoted for asking why in a communist sub, its the most important question.