TL;DR Because that is how they have been designed. You can use a KDF to derive keys containing at most the randomness present in the input key material.
I've noticed that keys for authenticated encryption primitives like AES must be unpredictable and uniformly random in order to be secure.
Well, they need to have 128 bits to have ~128 bit security as AES promises. In principle you could also generate, say, 112 bits of random key, then use 16 zero valued bits to make a 128 bit key. Most API's only accept bytes - i.e. multiples of 8 bits, so that would be 14 bytes random and 2 zeroed bytes for a 16 byte key. AES, in principle, doesn't require well distributed keys.
I've removed this part of the answer it has been pointed out in the side channel chat that this is not the case. That said, the known related key attacks on the key schedule won't directly work as they assume that specific attack requires a specific change in the supplied keys. However, without further analysis we cannot say that the result is secure; more information can be found here.
IV values and seeds for PRNGs also have to be unpredictable and random.
Not necessarily. IV requirements are different for each mode of operation. For instance, CBC requires an unpredictable IV (which generally translates into a randomized IV). CFB requires a nonce that is the same size as the block size. CTR mode can use any sized nonce, as long as the counter block doesn't repeat. GCM mode usually uses a 12 byte nonce. Nonces - numbers used once - can be randomized, but they may also be a serial number, for instance.
My question is: How those unpredictable and random values are different from predictable values that contain whole english words, for example (like verysecretkey123456)?
They are generally generated by a well seeded secure random number generator (CSPRNG or DRBG). Or they are derived from other information. For instance, they may be created using key agreement, usually followed by a key based key derivation function (KBKDF).
Keys may indeed also be generated from a password using a PBKDF. Those functions also require a salt and work factor (and possibly other parameters) to mitigate the risk of an attacker guessing the password. Unless other countermeasures are present the scheme could still be insecure as passwords are generally easy to guess.
"verysecretkey123456" is generally not considered a key, it would be considered a password or passphrase. The fact that it is a string rather than binary is enough of a hint for that.
I assume from the perspective of the algorithm it doesn't matter (as long as the key length is correct),
Correct, although AES requires a bit string as a key, so to use it at all you'd first have to encode the password or passphrase. Note that e.g. DES keys have parity bits in the encoded key, so not all keys take just randomized bits. More modern ciphers than DES - including AES - are generally designed to take fully randomized keys though.
but attackers can guess predictable keys/passwords more easily because they try weak keys/passwords first. Am I right?
Yes, that would generally be called a dictionary attack when they try specific words. Of course, they may test very small keys separately, and dictionary attacks can be augmented by other tricks.
And uniformly random only has meaning when the keys are generated more than once? If so, then what will happen when we use non-uniformly random keys?
No, as mentioned in the other answer uniformly random talks about the generation process, not the outcome as such. If you have non-uniformly random keys then there is a chance that an adversary can guess them - at least more likely than if the keys were uniformly random.
As per the Kerckhoff principles we generally assume that attackers know how keys or passwords are generated, so from that they would be able to guess the distribution.
