That delusion is so common that I suspect it must have a name, but I don't know what it is.
But the better a solution appears, the more a person who sees the solution believes it to be obvious, and therefore also believes that, had they tried to solve the problem, they'd have come up with this "obvious" solution straight away with no effort.
Meanwhile, the person who actually solved the problem could only come up with the perfect simple solution after a lifetime of study in the area to the point that you'd call them an expert or master, and after agonizing for a long time over this particular problem.