Why is celebrating the Prophet's Birthday considered an innovation (bid‘ah), even though it is merely a celebration unrelated to worship, similar to birthdays, knowing that an innovation is anything newly introduced into the religion as an act of worship?

Celebrating the Prophet's Birthday is a "religious holiday" observed by those who do so as an act of drawing closer to God, and this is for several reasons: Firstly, the celebrants do so out of love for the Prophet, peace be upon him, and love for Allah and His Messenger is one of the greatest acts of worship. Therefore, whatever is done for its sake is an act of worship, even if it were good, the Companions, who loved the Prophet, peace be upon him, more intensely, would have done it. Secondly, seasonal celebration makes it a holiday, and holidays are among the rituals of religions; they are part of the divine laws and rites, and not merely customs. Legislating holidays that Allah has not prescribed is considered a fabrication and an innovation. Thirdly, the Prophet, peace be upon him, replaced the two Jahiliyyah holidays with the two Eids of Fitr and Adha. Had celebrating a holiday been a custom unrelated to worship or imitating disbelievers, he would not have forbidden them. So, how about something done as an act of drawing closer to God or associated with it, when he, peace be upon him, said: "Whoever introduces into this matter of ours that which is not of it, it is rejected."