Because bigger seedheads cost more. Crops have lots of spare energy, because they&#x27;re looked after by human farmers. This isn&#x27;t a problem for them.<p>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.<p>As an example, look how badly most of our engineered crops do in the wild; there&#x27;s massive selection pressure <i>against</i> the engineering that makes them useful to us.