It's more about feel than sound. A really well crafted bass just plays right. It's just like and extension of your body. Until you've experienced it, it's really hard to explain.
I hear what you're saying, but ... I own Fodera, F Bass, Drozd, Sadowsky, Lakland, LeCompte, Yamaha and several Fender basses, so I've experienced it. There's no reason why an inexpensive bass can't "feel" just as good as a custom bass. I feel different when playing my Foderas from when I'm playing my Fenders, but it has much less to do with playability than ego. I owned an EB MM Stingray 5, a very nice bass. But I much prefer the "feel" of an SX jazz I played that cost about one tenth as much.
There's a huge psychological component to believing that a custom bass is more playable than an inexpensive one. Do you think the better feel is due to better programming of the builders' respective CNC machines?