This^

OP, you are completely and absolutely wrong. The private keys ARE NOT randomly generated characters. They are actually 256 bit numbers. This means that the total number of possible private keys is 2^256 - 1, which is a ridiculously large amount of possible private keys. Those private keys are converted into the characters that you see through a process known as Base58 Check Encoding (https://en.bitcoin.it/wiki/Base58Check_encoding). You cannot just throw random characters together to get a private key because it will probably fail the Check part of Base58 Check Encoding. The Check is the first four bytes of a SHA256 checksum of the private key.