The main reason is that most solar arrays are set up in such a way that they require the grid to operate. If you want to run your array without the grid it's a different type of system and requires additional hardware (transfer switches) to ensure that you don't backfeed power to the grid, creating a safety hazard for people working on the lines. Storage is also a necessity if you're operating off-grid.

Newer inverters also use a type of communication over the power line where the utility can among other things tell the system to curtail export power if there's too much of a surplus on the grid.