Why not use "were caught up"?

The context below says " rich children were caught up by poor children at the age of 6 in IQ". I don't understand why "were" can be neglected.

Context:

It turns out that babies who had low IQs at 22 months and were born to richer, better educated parents caught up by the age of 6 with children who started with high IQs but whose parents were poorer and less educated. By age 10, the children in the higher socio-economic group were forging ahead on intelligence tests while those in the lower socio-economic group fell further and further behind (see diagram). "It shows that the social is trumping the biological," says Marmot. "We can change that, and that's why I'm optimistic."