They've built up a brand over 30 years that's now very well-known. Walking away from that is hard when you don't have money to advertise the new brand with. Even if their current brand and image is off-putting to many, in politics, it's much better to be hated than to be irrelevant. A party or independent politician can only begin to change anything if the voters remember who they are first.The Greens need a rebrand and better strategy to not give the media and other parties any ammunition to go after them.
As for the media and other parties, if they can't find easy ammunition, they'll just invent a lie to use instead, like their US counterparts did with Obama. And even if they didn't, it's very difficult to keep a party room of significant numbers so disciplined and clean that there's no ammunition to use against them.