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I still don't get why this happens, can someone explain it to me? If you see the transactions getting to the mempool, you don't need to stay long to see that there are a lot of them with way higher fees than required. I believe this is a problem with wallets, but isn't this something that should be sorted out already? I always check this myself before sending a transaction because I see this estimation isn't very well done by most wallets.
Basic reason is fee estimation algorithms are hard (you're trying to predict the future) and it's easier to just overpay when a tx is high priority.
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I think that apps want good app store /play store ratings.
If transactions are slow noob users might blame the app instead of the time of transaction. Or at least devs fear that is what noob users might do.
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Could be exchanges paying the miners to motivate them. Some exchanges like OKCoin use flat USD fee, $20 or so converted using the current exchange rate.