Objectively,
Because we would be avoiding the 2 jet lag times a year which have been proven to cause deadly (traffic) accidents.
Because the other alternative of always dst means sun only gets up at 10am in winter which can negatively impact mental health (especially for children and teenagers)
Subjectively,
Because some people actually enjoy summer nights.
Because getting up on time is easier with more sunlight. And waking gradually and naturally at 5:30 beats waking forcefully by alarm at 6:30