Because of the stubborn premise that the original music is worthy of mockery in the first place. Just because it is chosen for parody (selected due to popularity, or merely whim) doesn't
imply that the original wasn't credible to begin with. Of course, musical taste is subjective so you're free to form your own opinions, but I think many of us here would have no shame in listening to the songs that were parodied. It's not that we don't "get the joke", or can't laugh at ourselves, it's that there's nothing funny there at all.
Remember, WAY's lyrics aren't usually making fun of the song or the artist. They're often about something else entirely, just riding on the same melody. Hearing a familiar song out of context is part of what makes it funny, but it's also more likely to appeal to listeners that have already heard (and possibly
like -- LOLZ!!!11) the original. The artists that like the parodies are in this camp. You would assume they like their own music, but they also like the parody as it's own product.
So. Don't like the music? That's fine. But insisting that the rest of us are failing to understand our folly implies that we like a
parody of a song we
don't like, which is often just not true. And even when it is, we can enjoy the reinterpretation on the basis of it being clever in its own right, without being daft.

<< You seem to be fond of these, so this one's for you.