Yeah, but most of them play bolt ons too, some more than their set necks. Way off topic now, back to the real question....erm what was it again ?!?!?!
PS: EJ, SL,RS,JB, EC and PG all have Signature guitars that are bolt ons !
The EJ signature is complete and utter BS. They just made a replica of the vintage Strat that Eric Johnson had been using for years. It has nothing to do with design. The guitar was designed in the 50s by Leo Fender. Eric Johnson just happened to get famous while using one, and Fender wanted to cash in on that.
The exact same story applies to Jeff Beck and Eric Clapton.
Paul Gilbert has had multiple signature guitars, some of which were bolt-on, some weren't (the violin guitar and the reverse Iceman for example). He's also a big fan of the lawsuit Ibanezes, but these cannot be made into production models for obvious reasons.
As for Steve Lukather... well what were they going to do, make a series-reproduction of his legendary '59 Les Paul?
His current signature is little more than a standard MM Silhouette with his custom EMG set. I doubt he had a lot of input in the design. MM simply doesn't make anything other than bolt-ons.
Most signatures just aren't designed by the artist at all, they're just off-the-shelf designs with some special specs, that don't go much further than custom pickups, hardware and paintjob. And even if they do have some input in the design, they're often limited by what the luthier can mass-produce at an affordable price.
Even Les Paul didn't have all that much input in the design of the guitar.