Sure, and every time I use such a "custom" feature, I have to look up how it works. If markdown were less static, the whole language might be like that. That's fine for some uses, but not when the use is a simpler, more memorable way to generate html.
If Markdown were less static, you wouldn't have to find random implementation-specific documentation describing custom features - they'd be defined in the spec. You also wouldn't wind up in situations where two different Markdown implementations interpret the same Markdown text in two different ways, which happens surprisingly often.
Markdown text is therefore always written for use with a specific implementation, and can't realistically be used as a text interchange format in e.g. email or any other federated system with multiple rendering implementations.
