Partly because doing so risks that they might decide to invest in their own production instead, and therefore not buy any electricity from you at all which would result in loss of demand, and a reduction in overall electricity cost.
Like how rising a bus ticket fare by 10% means you will lose some customers because they decide to walk instead, so your profit increase will be lower than that 10%. Raise it too much, and almost everyone walks, and you sell no tickets.

And it’s a lot harder to build your own solar or wind farm if you are a person living in an apartment building.