Millions of people think that. It is an uninformed opinion.
No, it is an opinion informed by evidence.
Anyone reading the Quran without a pre-existing belief in Islam and its divine origin would conclude that it was most likely written by one or more 7th century Arab men.
In the same way that people deny Jesus, people deny Muhammad.
They don't usually deny the historical character of Jesus. They just reject the stories about magic. But yes, the two characters are quite similar in some ways. Like Jesus, some historians suggest that the character of Muhammad never actually existed, based on lack of archaeological evidence from the period around when he was supposed to have lived.
Either way, there are clear similarities between the two.
Probable historical character with some corroborative evidence, but not conclusive.
Zero evidence for any magical powers or divine connection.
Possibly never existed as an individual at all.
That is the reality of the situation.
It is so easy to make a claim. However, it is not so easy to back it up.
As religionists have been demonstrating for thousands of years.
Were Muhammad and his companions all "in it together" ?..or what?
Some might have been. Others might just have been following a charismatic leader.
Remember that if you take out all the claims about god, the Quran is just the record of a successful 7th century political and military leader from Arabia. Nothing remarkable about it.