It seems odd that the islands were unsettled, considering the large supply of fresh water and food on the islands. Was it luck, were the ocean currents or winds unfavorable, or is there some other explanation?
Remember that if they didn't know they were there they wouldn't know what resources they held. Thus, they need to find them before they can assess them.
And, since they are about 1,000 km to the East of Madagascar, and not in the direction of anything in particular (or not for a long way) it's reasonable to think they would have had to be found by accident, if at all.
Remember that the westward Austronesian expansion is thought to have been more of a coastal-hugging enterprise along the northern margin of the Indian Ocean, not a straight shot through the heart of that ocean.