Without arguing in favor or against the usefulness of HFT, the reason trading hours are limited is to increase liquidity during the specific hours that trading takes place. Liquidity is important for various reasons, in particular it helps reduce spread and thus there are better prices. Trading during other hours is possible, either during pre-market or after-hours, and there are even exchanges that enable trading on weekends, for example https://www.ls-tc.de/de/faq . During weekend trading, the spread is significantly higher.