And I kind of disagree. Most full-featured multipurpose systems absolutely have the tools for that to one degree or another. I think you're overgeneralizing from looking only at the class-and-level game sphere.
There is no mention that the system chosen is multipurpose. The majority of games are tailored to specific styles, tones, genre, and challenge foci.
If a DM came to me with a GOT pitch in a system that has elves, magic, and monsters in the core book and doesn't immediately hand or mention to me a list of houserules, homebrews, and/or additional book to alter the game...
...I am going to assume he or she has NO IDEA what they are doing despite wanting to play. And some people mention ask to be an elf to test or troll him/her/them.