Why do most plants produced many more seeds than will ever become adult plants?
It is a numbers game. Most plants produce more seeds than are
likely to become adult plants BECAUSE so many seeds fail to grow to
adulthood. If they did not overproduce, the species would die
out.
In fact, one way to measure the "effectiveness" of a plant's
seeds is to look at how many it produces. Plants that produce the
most seeds in comparison with the seed survival rate are those that
have ineffective seeds.
Plants that produce fewer seeds generally have more effective
ways of ensuring that the seed survives to produce the next
generation of plants. Some of the ways that plants use to increase
the effectiveness of their seeds are by protecting them with shells
(so that they are resistant to the weather), making them larger (so
that they have a lot of stored food to give them a good start),
making them taste good to birds (so they will be eaten and
deposited along with fertilizer).