Because simply being similar to a Les Paul is not enough to warrant automatic trademark infringement.
Gibson has gone after other companies in the past to mixed results.
When PRS launched their Singlecut model Gibson sued them for trademark infringement and initially won, but a year later the decision was reversed in a higher court and production was able to resume.
Gibson did have a win against ESP, who changed the shape of their Eclipse. And of course in the 70s Gibson went after Ibanez for important Japanese clones.
Agile, Vintage, etc already avoid trademark infringement. That's why their body shapes are similar to but not identical to a Les Paul. You'll notice Dean's Thoroughbred model wasn't included in Gibson's lawsuit. It's just the V and the Z, which have identical bodies to Gibson's Flying V and Explorer.
And keep in mind these trademarks just cover the US. In Japan it's not enforced, which is why you still see accurate Les Paul copies being built with Gibson's headstock shape, but US retailers cannot import them. That's why the Japanese Epiphones, and the Orvilles before them, used the open book headstock - Gibson had to compete with Burny, Greco, Bacchus, Edwards, Tokai, etc. In the West Gibson is protected from competition, so they can offer a compromised headstock shape.