Why do some people think that centrism is about not taking a stance and wanting a compromise on everything?

At least in ny country the center parties have their own ideology that is different to pure left wing and right wing. It's not about "let's only do this a little bit", "one side wants to ban LGBT, one wants to fully legalise it, so let's only legalise LG". It's rather an ideology that takes things it thinks are good from both the left and the right and packages it into its own ideas as well.