[q084yn39cptyth]

Others have mentioned the size of projects, which might be true.

Another factor is the incentive structure. There's immense pressure to obtain grants (due to indirect fund income) and focus on h factors increases this as well. The best case strategy given both of those incentives is to have large groups of people who each submit as many grants and papers as possible, and put each other on each submission. That way, even with a high grant failure rate each will bring some money in, and self-citations to a paper will be multiplied by the number of authors.