Just like we repeat a blessing on every cup of wine at the seder, why dont we repeat the Moitze on the Afikoman?

Shalom, Thank you for your question. Actually, you touch on an issue that has been addressed by great Rabbis for generations! (Well done on the great question!). Many early Rabbis actually believe that one does not need a new a bracha on the 2nd and 4th cups – because the blessings on the first cup covers the 2nd. After the meal we will have said birkat hamazon (grace after meals) and that creates an interruption which requires a new blessing on the 3rd cup. That blessing is good for the 4th cup also, according to this position. The Sephardim rule like this. This is totally in line with your question. The Ashkanazim follow the opinion that each cup stands out by itself, and requires a new blessing. This is because we preform a separate mitzvah on each cup – kiddush, telling the seder story, grace after meals, hallel. Each mitzvah requires a new blessing on it’s cup of wine. (Some offer other explanations – but this is the opinion of the Magan Avraham and Taz 474:1). The HaMotzi is not repeated on the Afikoman because the blessing over “bread” covers all bread eaten until grace after meals, which is said only after eating the Afikoman. Blessings.