Because software is more complicated. And more complicated software (in addition to the primary effect of being complicated) is further slowed by abstraction and indirection, which slows performance but doesn't increase functionality.
Software that ran on dos or windows 3.1 WAS fast. But it also didn't have much in the way of UI.
Software that ran on dos or windows 3.1 WAS fast. But it also didn't have much in the way of UI.