Before you go on a voyage of discovery, you need a minimum of the following:
A rough idea of the area you are going to explore and knowledge of the distance from the nearest supply outpost.
Enough supplies for the complete trip.
A vessel and a bunch of crews fit for the journey.
Reading on the internet it seems Polynesians had none (of course I might be wrong), except the part of the crew. Their technology was "primitive" compared to what Columbus had when he set sail---even then he knew(or believed) where he was going. It seems irrational for Polynesians to decide on exploration, with few tools at their disposal since the trip would be akin to a suicide mission. Did they decide to start wandering among the vast stretches of the Pacific Ocean or they sighted land by chance of fate
I talked about this in a separate post. Basically, the Polynesians had an exceptional understanding of winds and currents, and strategically planned their exploratory voyaging so that in the event that they didn't find an island the return voyage was easier and more favorable than the outbound one. Supplies enough for the voyage would be kept onboard.
As for vessel capability, you greatly underestimate the ability of the traditional Austronesian vessels. If nothing else, the very fact that the voyages manifestly occurred proves that the vessels were capable. You should look up the Polynesian Voyaging Society which frequently makes voyages throughout Oceana using only traditional methods and instrumentation.