"Reasons why not" are indeed a dime a dozen, but that doesn't mean they are useless. It may be a bit annoying to have to filter through the feedback that identifies the wrong risks, but we shouldn't use that as an excuse to ignore critical feedback. Some of the best feedback I've ever received that got my mind churning was from people who truly understood the problems and were able to identify "reasons why not" that I hadn't thought of.
On the contrary, it seems more likely that Musk would've not only thought about risks already for more than a couple hours, but that he would've consulted a lot of domain experts about such things before even deciding to take on such a project. Even the most optimistic business person isn't naive enough to take on something they see as a sure bet to fail. Not to suggest that there haven't been entrepreneurs attempting silly businesses before, but rather that one should pause and think about the possibilities of how something might work, before jumping to the conclusion that they already understand everything there is to know about a project, and that it definitely won't work.
As another comment mentioned elsewhere in this thread, a good entrepreneur's main skill is being able to figure out some unique/unorthodox way to make something work, and capitalizing on that. Now, that doesn't mean that everyone opposing these ideas are wrong or don't know what they're talking about, it just goes to show that a lot of people's default state is one of risk-aversion.
Three months ago, after the Bloomberg feature was released, he literally said that they had no idea what they were doing.
Also, there's a ton of companies with a ton of knowledge who have done things which failed spectacularly or have refused to do things that would work. Tesla's whole existence is sentiment to that.
Is Musk immune to making such mistakes? The consensus on his Hyperloop idea still seems to be that is infeasible.