There is nothing preventing other animals from becoming just as good at manipulating their environments, though. Raccoons have independently evolved dextrous hands very similar to those of primates, for example.
And I think there's a bit of anthropocentric bias creeping in to the way you're looking at this. It doesn't follow that, because the only current species with a technological society uses a pentdactyl hand as their primary means of manipulating tools, a pentadactly hand is the best or only method of manipulating tools. It could simply be that, because we evolved from primates with pentadactyl, grasping hands, these hands were the most appropriate organ to turn to tool use.
I'm sure you've seen people on TV without working arms who've impressed with the intricate tasks they can accomplish using mouths and or feet. These are people born into a species which normally relies on their hands, and yet they're capable of learning, with practice, to do many of the same things without. In a species which never relied on hands, there's no reason they couldn't manage to accomplish complex tasks of tool-making and tool use.
To those who say that such a society would have to leave traces, it depends what level of technological sophistication we're discussing, how wide-spread a population. and how long they were here on earth for. We're constantly reminded that humanity is but a blip in geological history, and the period in which we've radically expanded in population and started building megacities is but a blip in human history.
Whilst it's true that there are Cambrian rocks that are yet to be subducted and aren't buried in places palaeontologists don't dig, what proportion of all the rocks dotted around the Cambrian world do they represent? I'd imagine a very low one, please correct me if I'm wrong. Then consider how many of the rocks around in the world today will bear clear signals of a technological society, a figure that I'm sure would be less if we looked back 2,000 years or more in history.
Now, I've no idea how likely another technological civillisation preceding us would be, and it's only idle speculation supported by no evidence, but I don't think it's unreasonable that Cerebrosaurus sapiens could have spent a few thousand years building cities, fighting wars and composing poetry before being wiped out by that big asteroid, without leaving any trace that we've noticed so far. Especially considering the entire length of their putative civillisation would fit into the error margin for the date of the end-Cretaceous event 100 times over.
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