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.
It's possible there's a US copyright claim, but on a 1954 design, you would have to have registered it, marked the works with the copyright (on at least most of the copies), and timely renewed. There's also, IMHO, a solid question of if US copyright applies to the shape of a guitar. If they had a strong case, I think they would have tried to enforce on it in 2009 when they tried to enforce on trademark.
[1] https://www.musicradar.com/news/guitars/fender-loses-guitar-...
The shape might not be purely functional—and that seems to be the basis for their attempted lawsuit.
https://guitar.com/features/opinion-analysis/how-les-pauls-l...
The predecessor was a single string on a piece of actual rail and two spikes, amplified by a telephone receiver in the mid-late 1920s.
in Germany, Fender recently won a default ruling because a chinese counterfeiter didn't even show up in court, and they're now using that to go after anyone selling in the EU, even though that's not really what that case win means. But private equity is going to private equity.