This is not true at all. First, many insects can fly very fast. Even honeybees, which are notoriously poor fliers in the insect world, can reach up to 20mph, carrying a load of pollen or nectar. Dragonflies can reach over 60mph, and some flies may be able to reach upwards of 80-90mph. Second, many of these insects have excellent visual collision avoidance mechanisms and the strongest fliers have exquisite control over their flight. There is a lot of literature on this topic.