The costs of keeping them current adds up surprisingly quickly.
In fact, it adds up so quickly that my company has a standing policy in place that we will do _whatever_it_takes_ to have any pull requests we need in an upstream project integrated in order to avoid us having to maintain a fork.
What this means in practice usually is that getting the pull request accepted becomes the primary focus for one of the senior engineers for the 1-2 weeks it takes to get it into a shape that the project maintainer can work with.
As you can imagine this is also expensive, but it's a fraction of the cost of maintaining our own fork of the project.
So - here's my offer to Libré, Free and Open Source Software maintainers everywhere:
I will _gladly_ pay the cost of 1 week of a senior engineer's salary to have you accept my pull request if you are prepared to either:
1) do the quality control work yourself or…
2) hold the hand of one of my junior engineers while they learn how to do it.
I too would gladly pay one dollar for a hundred dollars bill.