Each neurone is itself a complex combination of chemicals cycles; these can be, and have been, simulated.
The most complex chemicals in biology are proteins; these can be directly simulated with great difficulty, and we've now got AI that have learned to predict them much faster than the direct simulations on a classical computer ever could.
Those direct simulations are based on quantum mechanics, or at least computationally tractable approximations of it; QM is lots of linear algebra and either a random number generator or superdeterminism, either of which is still a thing a computer can do (even if the former requires a connection to a quantum-random source).
The open question is not "can computers think?", but rather "how detailed does the simulation have to be in order for it to think?"
