Because:

1. There are lots of things that compilers load into their memory that aren't actually source code. A memory exploit turns non-source data into executing-in-the-compiler code.

2. Depending on the language semantics, a memory exploit can allow substantially higher privilege than just being loaded as library code. Latent malicious code that never gets called into never becomes active, but if you can exploit a weakness in the compiler you can make your code execute at any time you'd like instead of relying on the main application calling in to your malicious library.

1. What would this be? 2. Good point, but for most purposes I think exploiting some aspect of the build system or the language that moves the exploit code or causes it to be executed before main would be easier than exploiting a memory issue in the compiler (I agree though in principle).