Because solar panels are unreliable and cost too much. Unless you install a boatload of batteries you will always need those transmission lines.
My point is instead of creating giant solar farms and transporting the power to the consumer it makes far more sense to locate the panels at the consumption site. Regardless of the reliability or storage requirements it makes more sense to put the panels where they are ultimately used. Financially it would be far cheaper and functionally it would be more reliable.
Solar is very reliable here in Colorado given the number of sunny days and if you put aside cost does provide independence from the power grid. There are enough taxpayer incentives in place to see a ROI in roughly 12 years. If panels last 25 years without major maintenance that would be a significant savings.