Edit: I found a forum post that goes over it:
First I need to say I don't know how to program, let alone know how a PS2 or its emulation works. But I always had this question: Why does PCSX2 has the Interlacing option at all? I know the image of
forums.pcsx2.net
Basically, other systems have standardized interlacing techniques that are easier to counteract behind the scenes, PS2 is a hodge podge of however the developer wanted to do it on a per game basis and the de-interlacing work is therefore not as straightforward.