It's an investment. The first few patches from a new person will likely require so much hand-holding from the maintainers that it would have taken them less time to just write the patches themselves.
As that new contributor learns the ropes, though, that cost goes does. Eventually they hit break-even where they are adding more value to the project than they cost. But that requires the person to stick around long enough to get there. There's a pretty high chance any new contributor will lose interest and go do something else.
So whenever a maintainer gets a new patch from a new person, it's always an open question as to whether taking it in will ultimately be a win for the project. Unless you can predict the future, you can't tell if that investment will get amortize out and yield something useful.
I have 1 patch in the git kernel. I barely knew c when I did it but I did tons of research on what to call and how. Junio (maintainer) took my barely workable patch and told me what to fix. Let me do it, then pulled it in. Amazing experience in a very scary environment, eg non-minimal chance Linus tears into you.
As that new contributor learns the ropes, though, that cost goes does. Eventually they hit break-even where they are adding more value to the project than they cost. But that requires the person to stick around long enough to get there. There's a pretty high chance any new contributor will lose interest and go do something else.
So whenever a maintainer gets a new patch from a new person, it's always an open question as to whether taking it in will ultimately be a win for the project. Unless you can predict the future, you can't tell if that investment will get amortize out and yield something useful.