What do you do to your mouses to make them fail so quickly? Are you throwing them randomly accross the room?
Use cats. For all great Logi ergonomics, they fall prey to cat hair; and pads cover the screws, so they can't be reassembled pristine after cleaning. Still, M500s is the best.
Actually using the middle click button regularly.
They just fail. Particularly the office mice don't last very long for gaming (orders of magnitude more travel and clicks)
Some Logitech mouse switches have been known to fail in normal use.
At least one person has put together a good overview of what they think is happening: https://www.youtube.com/watch?v=v5BhECVlKJA (details in video description)