That kind of AI would be more busy harvesting new worlds than trying to care about an inferior species like ours (and they would likely not even be in the vicinity so no reason to travel up to us either).
Other possibilities:
- The sentient / strong AI may have (developed) FTL travel / signalling capabilities which from our current understanding does not seem to be possible.
- The AI may have sent out scouts millions of years ago into multiple directions. The closest scout only needs to decide to conquer / harvest our planet.
- A combination of the above, for example: FTL signalling and activating scouts where needed.
favorite quote about the possibility concerning all of this Arthur C. Clarke — 'Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.'