The small studios (or one-man armies) are the ones using custom stuff because they have to do it all anyways, no biggie having to do a little leg work to import stuff.
When you're a bigger studio, having to coordinate and support multiple teams with different workflows, the engine and the tools it provides play a much bigger role. The studios with such scale either build something that works for them, or use some existing one provided by others.
The small studios (or one-man armies) are the ones using custom stuff because they have to do it all anyways, no biggie having to do a little leg work to import stuff.
When you're a bigger studio, having to coordinate and support multiple teams with different workflows, the engine and the tools it provides play a much bigger role. The studios with such scale either build something that works for them, or use some existing one provided by others.