Writing an unpolished programming language isn't that much work in comparison to writing a complex virus. Especially low level languages where instructions map pretty closely to the CPU instructions are easy to create.

I think it makes a lot of sense to write a custom programming language/compiler because virus scanners tend to use fingerprints to recognize dangerous pieces of code. So you want a compiler that deliberately obfuscates the code it writes and also outputs instructions in such a way that it avoids triggering known virus scanner fingerprints.