Simply because people are still mining. All the miners believe that Ether will return back to its $400 days soon. It’ll take a while before difficulty starts decreasing, that only happens when the price drops down significantly for a long time.
Read moreWhy is Ethereum mining difficulty so high?
If the number of miners increases, the network hashrate goes up. The effective block find time becomes lower than the preset value . As a result, the network gradually increases its difficulty, that is, the difficulty of a problem that miners are solving.
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