Well, it's true, they do. And they will continue to do so. It is one of the injustices in a gardener's life. But understanding the hows and whys of the situation can help in weed management. One reason weeds are so persistent is because of the soil seedbank, the collection of weed seeds that have been produced in the past, sometimes in the distant past.
Most soils contain millions of weed seeds.
Researchers in England once dug up and sifted through the top few inches of soil on a plot of land. And this is only a partial list. This not surprising when you consider the numbers of seeds that one plant can produce in a season. For example, a single lambsquarters plant can produce 39,000 seeds in one season. A garden plot in which the lambsquarters plants were not controlled for a single summer would have several million seeds deposited in the seedbank in one year.