personally, given all the evidence at hand, I think it's because that's the way the book was written.
lacked the ambition of the Big Folk,
with this one point being made, we remember particularly their lack of 'ambition for power' comes into play.
they had enough 'ambition' of a certain kind... like for second breakfast and onesies. they could simply care less about 'power and control.' Gando alludes to this with his 'especially (military) leaders' comment... but I think we can put a more pointed pin into it.. 'power and control'.
This makes, in many respects, the
hobbits much like
Ungoliant,
Shelob, the
dragons (in some ways) and others, who had absolutely no use for 'the rest of the world and their issues/problems', just wanting to satisfy themselves, and otherwise be left alone. (
Bombadil is among them.)
The big difference is what shakes them out of their self-centeredness.
With Ungoliant, Shelob, dragons, etc. the only thing shaking them was personal gain.
With Bombadil, nothing could shake him out of his narrow little life.
With
Bilbo, it was a little selfish as he didn't want to be considered timid... and did in the end desire to help the
Dwarves, and have an adventure.
With
Frodo, the need was the protection of other people.
With
Sam, it was his love for his master and overarching attachment to his role of 'servant/protector'.
Then we get to
gollum... to whom we can attribute nothing other than a desire for isolationism and self-preservation. Even worse than Bombadil, who was happy with just one person in his life, gollum wanted only what he could eat. He didn't even have the ambition of Ungoliant - (to consume all light) - a
dragon - (self-gain as bribed and fed by
Melkor, or to gain and sit on a hoard afterward), or a
hobbit (to enjoy
food and cheer).
In my view a GREAT point is made in that
Sauron was reforming. He was beginning to reassert his will for power and control over the physical world again. For a time, he too thought
the ring had been destroyed.
The ring no doubt began to exert its effort to seize opportunities as Sauron's initial strength grew again...the two being integrally and inseparably connected. Even though Sauron still wasn't aware of its existence, it doesn't mean his power wasn't active within it from the beginning - (which was the "nasty" power the ring had, though it no doubt had some power of its own, as the other rings did, especially the Elven rings.) The whole purpose of a ring was to "enhance" the power of the bearer in some way...meaning it had inherent power of its own. And Sauron also put a great deal of his own power into
the One.
Anyway, somewhere in the above ramble are my thoughts about why gollum didn't fade. His world was the most dismal, freakishly alone existence imaginable, he was completely without desire other than surviving hidden, and he rarely used it, never using any of its inherent power other than to be invisibile so he could sustain himself as effortlessly as possible.
Indeed he was a 'miserable creature'. What a great place for the ring to hide itself until 'the appropriate time'.
A bigger question for me is, did the ring 'know' it was going to end up hidden under a mountain for countless years when it revealed itself to Deagol, or was it hoping for a more illustrious career until Sauron returned? Did it 'know'
Smeagol would murder Deagol and end up forced to go in hiding? (we remember he was sneaking about, and 'driven out'... he did not go hide initially by his own choice.)
Ah, there's nothing like the Rings to build speculative and unanswerable questions.
That's what happens with pseudo-mythology and fantasy.