[_ah]

I had a meteorologist explain this to me once: weather forecasting is highly complex, near to an art form. It's not yet possible to model the forecast with precise enough rules that it can be done by machine within a reasonable compute budget. In addition, as others have noted, variations in topography can create many microclimates, so that the forecast for a given region is actually the average forecast for all the area within that region.

Very accurate forecasts for a specific spot are possible, but today require a trained meteorologist. In practice, the only time people are willing to pay for this expertise are when they're running an airport, planning a war, or launching / landing a spacecraft.

Perhaps in the future we'll have enough compute power on demand for anyone to accurately model on-demand forecasts for specific spots.