Because we're slaves to things starting at the same time throughout the year (work, school, etc) with timezone changes being the only way of taking advantage of the extra sunlight during the day.
I'd personally go for UTC (GMT) all year round and adjust the times we start work accordingly throughout the year, with as many countries as possible using it. Just start at an earlier time during the summer months instead of waiting for daylight saving time to do the job for you!
Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity,[7] modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited and often contradictory.[8]
One more hour of daylight means that we use less electricity on lights etc. which means we use less CO2.. You might think "Meh, so what? It's just a light bulb?", but since it's everyone on earth who saves this one light bulb, it will mean we are saving tonnes of CO2 outlet worldwide..
So not completly irrelevant