While all of these answers are mostly true, you have to go back in time. Darwin called it the abomniable mystery. Flowering plants and insects co-evolved rapidly roughly 150 MYA. So prior to flowering plants, there were few plants and insects and they were mostly generalists. The rapid expansion and explosion of insect diversity is deeply entangled with the explosion of diversity in angiosperms.
While all of these answers are mostly true, you have to go back in time. Darwin called it the abomniable mystery. Flowering plants and insects co-evolved rapidly roughly 150 MYA. So prior to flowering plants, there were few plants and insects and they were mostly generalists. The rapid expansion and explosion of insect diversity is deeply entangled with the explosion of diversity in angiosperms.