To proclaim the truth that there’s only ONE God and to call mankind and jinn kind to worship only ALLAH TA’ALA without ascribing partners to Him and to submit to ALLAH TA’ALA.

“I (Muhammad) have been commanded only to worship the Lord of this city (Makkah) Who has sanctified it and to Whom belongs everything. And I am commanded to be from among the Muslims (those who submit to Allah in Islam).”

Surah An Naml 27, verse 91

2. To deliver glad tidings and warnings and reminders.

“And with truth, We have sent it down (I.e the Qur’an) and with truth, it has descended. And We have sent you (Muhammad) as nothing but a bearer of glad tidings (of Paradise, for those who follow your Message of Islamic Monotheism) and a warner (of Hell-Fire for those who refuse to follow your Message of Islamic Monotheism).”
Surah Al Isra 17, verse 105

“And warn them (0 Muhammad) of the Day of grief and regrets, when the case has been decided, while (now) they are in a state of carelessness, and they believe not.”

Surah Maryam 19, verse 39

“So remind them (O Muhammad), you are the only one who reminds. You are not a dictator over them. Save the one who turns away and disbelieves Then Allah will punish

And the big reason is that the people are can’t see Allah s w a

That’s why Allah sent their messenger