Why are brain wallets not secure? People keep saying it, but explain to me why? Everyone keeps saying brain wallets are insecure. Can someone explain to me WHY? 

What if I had a brain wallet that is 200 characters long which is the concatenation of all of my family members names along with punctuation's and numbers? How about that?

And how long is a private key? I don't see how a brain wallet is insecure if you come up with something ONLY you can know.
It's not to do with length, but entropy. Humans are terrible at creating randomness, even what you think is hard to guess could in all probability be guessed with enough iteration. 

Family names for example are easy to guess, there's a finite number of them and a vast portion of names are from a set of only a few hundred. 

What's the chance of you remembering what names are, the spelling, punctuation and ordering? Fairly low I suspect, humans are also terrible at remembering explicit data like that.  

As soon as people start trying to be clever, the probably shifts from theft to irrecoverable loss. Just use the cryptographic keys the way they are supposed to be used, 256 bits of entropy created with a CSRNG.