How Can You Not Know It Is Superman?
The similarities between Kent and Superman is only a pair of non-prescription glasses, and a suit of clothes, therefore how can one not know this is the same person? Is everyone in the Daily Planet that gullable and yet able to investigate stories? What do you think?
The reason they are "Blind" to the similarity between Kent and Superman is the simple reason that Kent is considered what we call "Nerd", I would go further and even say he is "Goofy" and "Boring". Nobody can not even comprehend seeing a similarity between the two. Kent hardly talks and is shy while Superman is so self-confident.
Nevertheless, it is Hollywood!
Other then the first few times you meet some one you not little of their physical appearance unless some thing is very different and that even can take time to sink in.
How we perceive some one is much more then the physical and we often distort what we see by the persons personality and that is why it works so well. If noted it is immediately dismissed as he is a puny nerd.
Another consideration is that in one comic it offered the explanation that his glasses is made from that of the glass from his ship as a child, giving of a weak telepathic signal that prevents others from realizing Kent and Superman is the same.
Still, his demeanor and self confidence does offer for a good explanation. That Kent and Superman are just so different, the latter coming off so unassuming while the former stands out so much.