Why don't block ciphers consist of a simple XOR operation with key like Stream ciphers? If the construction of stream ciphers is already secure why does the construction of block cipher involve pseudo permutations and complex operations that makes block ciphers less faster? 
Is a construction of a block cipher defined as just Xor-ing a block of bytes with a block of key bytes (removing all permutations and other operations) not secure?
If the construction of stream ciphers is already secure why does the construction of block cipher involve pseudo permutations and complex operations that makes block ciphers less faster?


A stream cipher consists of more than just a single XOR operation. They first have to spend time generating the key stream from the key and nonce/seed. They actually use many of the same operations and take about the same amount of time as a block cipher does to do so. 

For example, Salsa20 is a stream cipher that uses addition, rotation, and XOR to generate the key stream.



After the key stream is generated, it is combined with the message via XOR. 

The stream cipher is only secure if the key stream is unpredictable. The complexity and time goes into making the output unpredictable.


  Is a construction of a block cipher defined as just Xor-ing a block of bytes with a block of key bytes (removing all permutations and other operations) not secure?


Yes, such a construction is insecure. Suppose that the adversary knows a single plaintext message. They can simply subtract out the known message to retrieve the key, which allows them to subtract out the key from any other ciphertext to recover the plaintext message. 

$c \leftarrow m \oplus key\\
key \leftarrow c \oplus m$

Unless your messages are the output of a random number generator and have 8-bits per byte of entropy, then they are susceptible to a known plaintext attack. Any meaningful message will not have 8-bits/byte of entropy, and so you have to assume that the adversary already knows the plaintext messages when designing a cipher.

Additionally, assume that you have two messages $m_1$ and $m_2$ that differ by only a single bit. When you XOR the key into them, the only difference in the resultant ciphertexts is at the place where the messages differed by one bit. A block cipher should produce a random permutation of message blocks, and this is clearly non-random behavior. A block cipher needs to spend time creating diffusion, where flipping a single input bit flips approximately half of the output bits.