NATO allies are unlikely to send more advanced jets to Ukraine because doing so could be seen as a hostile act toward Russia, which has a close relationship with Ukraine. Furthermore, NATO allies are not obligated to provide military aid to Ukraine, as Ukraine is not a member of the alliance. Additionally, sending more advanced jets could increase the risk of escalation of violence in the region.
This could be detrimental to the stability of the region and could even lead to an all-out conflict between NATO and Russia, which is something NATO would like to avoid. Finally, sending more advanced jets could also cause a rift between NATO and its members, as some members may not be comfortable with providing military aid to Ukraine.