Humans are proof that machine intelligence can be improved quite a bit. We are just complicated machines, no?
1. We don't know all the mechanisms that a brain employs to achieve intelligence. We see billions of interconnected neurons and we assumes "Yea, this might be generating intelligence".
2. We don't know if we are already at some fundamental limits of intelligence. For example, you can see may instances in nature where a pattern emerged that maximizes some sort of efficiency. (Like Honeycomb pattern). So, the end result of this will be that, even if we transfer the process by which our intelligence work to a machine, it will have the same performance as an average human brain...
Machines are the tools we make to aid the above.