Why would you personally need the entire internet to receive a fix?

I'm thinking of something on the order of Heartbleed. Sure, you fix it in your own servers. Are you sure you're ok with the entire Internet being vulnerable for two months? You don't have any data stored on servers that are outside of your control?

It's handy if you run a service and the internet runs clients you didn't write to access said service. (or vice versa)

Also handy if the internet is running a DDoS reflector and you're being targetted.

Otherwise, usually no sense of urgency for fixes I did for me/my employer and want the rest of the world to benefit. My problem is solved now, everyone else can get it when it ships.

Running a fork is a lot of work. You need your fixes upstreamed so that you don't need to backport other people's fixes

For a couple months? Not a big deal

Nobody said doing it yourself was neccesarily easy. Its just an option that is there.

You don’t need to backport other people’s fixes. You only need to re-merge your patches into updated versions of the upstream (aka vendor branch), which usually is straightforward.

Maybe you mean that if there are many people like you, they’d want to integrate each other’s fixes. But then you’d probably have the combined manpower to start maintaining a true fork.