It's become a thing because saying, "do your own research" is infinitely easier than investigating the current accepted body of knowledge to risk wasting months or years of your life only to find out that the current accepted body of knowledge was already correct.

"Do your own research" is the "trust no one" of the 2020's. Anyone who says it simply wants you to go away and isn't interested in debate and is unwilling to accept any information that might imply that they are wrong. If someone says "do your own research" to you and you are not both currently scientists working in the same field then it is safe to ignore them and everything they have ever said (and quite possibly everything they ever will say).