Why do most trees bloom before the leaves open?

This can be easily explained. Flowers need to be pollinated, and insects, for example, bees, do this. If the trees bloom after the leaves appear, then it would be more difficult for insects to notice the flowers and pollinate the maximum amount, since the leaves would cover the flowers and would not attract insects. Also, if pollinated by the wind, the leaves would interfere and retain the pollen. This is especially true for those trees in which flowers are collected in inflorescences. And in the absence of barriers to insects and wind, there is a great chance of pollination of all flowers and inflorescences.