Because if kafir simply means non-Muslim then these verses wouldn’t make sense:
The kafir from the People of the Book and the polytheists would not want you to receive any blessing from your Lord, but Allah selects whoever He wills for His mercy. And Allah is the Lord of infinite bounty. (Qur’an 2:105)
Indeed, they who committed kufr among the People of the Book and the polytheists will be in the fire of Hell, abiding eternally therein. Those are the worst of creatures. (Qur’an 98:6)
Yet they are not all alike: there are some among the People of the Book who are upright, who recite Allah’s revelations throughout the night, prostrating in prayer. They believe in Allah and the Last Day, encourage good and forbid evil, and race with one another in doing good. They are truly among the righteous.They will never be denied the reward for any good they have done. And Allah is well aware of those mindful of Him. (Qur’an 3:113-115)
And furthermore:
Those to whom We gave the Book recognize it (Qur’an) as they recognize their own children. Those who have ruined themselves will never believe. Who does more wrong than those who fabricate lies against Allah or deny His revelations? Indeed, the wrongdoers will never succeed. (Qur’an 6:20-21)
But when Our enlightening signs came to them, they said, “This is pure magic.” And, although their hearts were convinced the signs were true, they still denied them wrongfully and arrogantly. See then what was the end of the corrupt! (Qur’an 27:14)
Because it is the Way of Allah:
Whoever is guided is only guided for his own soul. And whoever errs only errs against it. And no bearer of burdens will bear the burden of another. And never would We punish until We sent a messenger (to make the truth clear). (Qur’an 17:17)
And if We had destroyed them with a punishment before him (Muhammad), they would have said, “Our Lord, why did You not send to us a messenger so we could have followed Your verses before we were humiliated and disgraced?” (Qur’an 20:135)
So anyone claiming that Allah is going to punish random confused non-Muslims really needs to make a good argument.