You have to look at the reasons why this behavior is occuring: Viruses get less harmful if it improves transmission. E.g. if you are more active and have a high

You have to look at the reasons why this behavior is occuring: Viruses get less harmful if it improves transmission. E.g. if you are more active and have a higher rate of contacts, even if infected and sick with a certain strain, that strain has an advantage. With Covid, most of the transmission occurs before symptoms appear, meaning that the severity of the following illness is irrelevant to transmission. In that case, the virus has no advantage from being less harmful. Also, simpler changes are preferred over more complex ones. Delta probably just has a mutation that allows it to reproduce faster after an infection, meaning that both infectiousness as well as severity of illness go up at the same time. Mutations that increase infectiousness but lower severity are usually more complex, so less likely to occur, especially if not selected for.