There is so little that you need to write for most GM projects that they don't see the value in code reuse.
When you get up into the 1000's of lines of code in other languages, it becomes a bit of an issue.
Speak for yourself. GML might be simplified but its hard to claim that "most GM projects" don't need many lines of code. Are you insane?
D&D is for beginners or for very basic objects in a game. Most people won't even touch D&D after a little while of using GM. I understand you can get away with writing less than an industry standard language might require, but your games must be very simple if you don't have hundreds of lines of code at least. Dear me.
There are many robust engines people give away for free on the GMC, some with royalty free DLLs.
I was speaking in relative terms.
From personal experience, I've never topped 2000 lines even in my largest GM projects, but hitting 10,000+ in Java/C/C++/etc isn't out of the ordinary.
No need to be so defensive.
Unless things have drastically changed since my active time here, most projects aren't very large and code rerolling isn't that much of an issue.
In other languages it is. People have dedicated their lives to C, C++, and Java optimizations. Attempting to rewrite their algorithms would be nothing but a waste of time.
Of course there are exceptions, but you can't generalize off of the edge cases.