Why did Facebook buy Instagram and Whatsapp? Why did Google buy Waze? Why did Volkswagen buy Audi and Skoda and Seat and Bentley and so on?

Might as well ask why companies like money.

Sure, but the car industry is pretty old and there's plenty of competition still.

I’m not sure about plenty when you actually look at it, but in any case it’s because of government intervention.

Think about it for a second mate: competition leads to winners and winners don't like competition, they like monopolies.

And no, the car industry has almost no competition. It's an oligopoly with very few players and a notoriously hard industry to get in.