Actually, if you have a garden, you find that as different plants come into flower, the bees and other insects congregate on them. Many herbs are flowering right now, and are covered in insects, but the blackberries are turning to fruit, so the bees are not on them, whereas before they were all over them. It's fascinating to watch this sequence go on, also I think bumble bees and honey bees seem to go for different flowers, possibly to do with size. But obviously flowers have evolved to attract insects, via colour, pattern, scent, and so on. (And some hybridized flowers don't attract insects, as we have taken the guts out of them).