Yom Tov Sheini

Question

I am curious about the way we keep this halacha today. I always learned we keep it because Chazal said we should as Minhag Avoteinu Be'Yadeinu. So I ask myself, if a Jew travelled to Eretz Yisroel during the time when we were using messengers to get out the timing of Rosh Chodesh, he would have kept one day, since he would have known when the Rosh Chodesh really was. Had he been Oleh Regel he would only have brought one Korban Pesach, not two since he was from outside Eretz Yisroel. Conversely had a Jew left Eretz Yisroel before Rosh Chodesh he would have kept two days since he would not have known. So why today when a Jew goes to Eretz Yisoroel does he not keep one day as the Halacha would have been when it actually was necessary. Thank you very much for your time and wonderful website.