Like a lot of people have said, Mew has nostalgic value. Back in the day when Pokemon came out, there were so many rumors about how to get Mew (remember the mysterious truck?) that were mostly all a lie. Mew was an actual legend, whilst Arceus can just be hacked these days. The mystical part has been taken away, although that's not necessarily Gamefreak's fault really.
So mew. Because it was actually a legend at school, and the guy who had one was universally worshipped or something
So mew. Because it was actually a legend at school, and the guy who had one was universally worshipped or something