Before home generation, everyone needed both. The cost of the infrastructure was amortized over all of the customers. As it turned out, rich people used more power, so by having a single charge just based on usage but that covered the cost of both usage and the grid, rich people subsidized poor people.

But the home solar came along, and the rich people started getting it. So now the power companies have a problem. They still have to maintain all of that grid infrastructure, whether you're connected to it or not, and they need rich people to subsidize it.

The could have simply split the bill up and put the infrastructure on the property tax bill and then just charge people for their actual usage, but that would be complicated and would reveal how much they pay for each part.

So instead they lobbied for laws that don't let you go off grid and force you to pay a minimum "connection fee" each month.