Because - until it makes its way through the courts - it’s not established that Fender has the rights to claim ownership of on the shape in the first place.
In the US, there’s three routes for that - design patent, trade dress and artistic copyright. AFAIK they don’t have a design patent. Trade dress is hard to prove association - would most people on the street say “yep, that’s 100% a Stratocaster” if they say the outline? Probably not. The shape isn’t separate from the functionality so artistic copyright hasn’t upheld either. The fact that Fender has not successfully enforced copyright concerns for over 70 years is also a sign that they never had IP protection on the shape.