Because bigger seedheads cost more. Crops have lots of spare energy, because they're looked after by human farmers. This isn't a problem for them. It is a prob

Because bigger seedheads cost more. Crops have lots of spare energy, because they're looked after by human farmers. This isn't a problem for them. It is a problem for weeds --- their energy budgets are already at equilibrium based on their environment. Allocating more of their energy budget to a bigger seedhead means less energy for doing other things, which would put them at a disadvantage. As an example, look how badly most of our engineered crops do in the wild; there's massive selection pressure against the engineering that makes them useful to us.