Why can't Niantic get the weather right?

At least there is no longer a stardust drought, with those boosted eevees at every corner

I've heard Accuweather mentioned as the source of the weather data. You might want to check that one out.

It's not Niantic's fault, do you really think they created a real-time weather detection system by themselves? They rely on some third-party system (just like they rely on OSM for map data) and if it is inaccurate for whatever reason, you can blame whatever local weather station is reporting the current state to that service. These problems are localized to the systems in place wherever you are living since satellite imagery is not enough to differentiate between rain clouds or just normal dense clouds. Most weather reports and forecasts rely on a combination of both satellite imagery and ground level sensors, and if a system is outputting the wrong specific weather for that moment in time, it is usually because the land services are not working properly or there are none at all where you live, which is the case for most non-metropolitan areas. Keep in mind that these systems like Accuweather try to give a real-time weather report on the WHOLE PLANET so, of course, there will be some giant margins of error in some areas, especially in sub-urban ones and countries where these systems (usually government funded) are neither properly installed nor technologically up-to-date.

The model used for generating Pokémon GO weather conditions may over-evaporate the rain during its fall through dry air in the desert. But in significant convective rain events (thunderstorms) rain should reach ground.

I have seen the same thing here though. If it is clear, the game often stays on Partly Cloudy. If cloudy, game can show Partly Cloudy or Rain. If rainy, it often switches to Cloudy. If it finally is windy, at best I get a visual swirl but no Windy status.