I spent two weeks delivering for Uber Eats and made $4.40 per hour

I don't understand the libertarian argument for allowing low wages. Let's say company X is paying low wages, which allows it to sell goods for cheap. If company Y tries to enter the market and pay higher wages, they won't get any market share, because their goods will be more expensive. So wages will stay low forever. Am I missing something?

Yes. Labour supply is not limitless. So in theory you could treat labour just as any other market good. Therefore, if there's an oversupply of labour, it will be cheap. If the supply is short, it will be expensive. There's many important caveats, and I don't claim to represent the Libertarian view fully, but that's the most important thing you are missing.