1. We don't know what intelligence is 2. AI can never be intelligent because humans are special (in various ways)
Of the two, I think that 1 is the more compelling to talk about. Let's look at state of the art Large Language Models (GPT, BERT, BART, T5, etc.) Everyone claims that they can't be intelligent because they're just cleverly predicting the next tokens. The most common failure mode of this is that they hallucinate - if you ask them to do something for you, they'll get it wrong in a way that kind of makes sense. There are some other more subtle problems as well like common sense reasoning, negation, and factuality. We could say that because of these problems they are not "intelligent". But why is that so important? Can we say with certainty that human intelligence is more than just patterned IO? If it is just highly tuned patterned IO with the environment, perhaps we have discovered intelligent systems, but they're handicapped because they're limited in their sensory perception (read: data modalities). And perhaps by combining several of these models in clever ways, we will end up with an architecture for pattern IO that is indistinguishable from human intelligence.
The naysayers claim that this won't work because we'll still end up with mere pattern prediction machines. But this starts to look like a "humans are special" argument.