Because it’s not just loading, it’s downloading data as well, and it has nothing to do with your SSD speed. If you use the sim completely offline, it loads much faster. But in order to get the benefits of live weather, better scenery, photogrammetry etc, it needs to download a lot of data, both when you start the sim and when you actually load a flight. I noticed that if I disable all streaming, even on my Xbox the sim starts in just a few seconds. So it’s not the local loading that takes long but downloading things from the cloud, and it depends more on internet and server speeds.