The answer is really simple, because they are living beings and all living things must grow before being able to reproduce and continue their species. By analogy with us, plants are starting to grow as children with the passage of time become men. Do not have children until they are big and adults. To complement this question Why do plants grow? , I present a video I found and I found that gives a good response, in which we can see some comparisons with other living beings and the characteristics of the variety of plants we have on the planet.
