If this human birth is taken as an opportunity to seek Salvation, then why at all did Niranjan give such an opportunity?

It was the compulsion of Niranjan. Suppose you are made a king. Then who will you rule over? On the animals, birds or on human beings. Certainly you would like to rule over human beings like you. For that, you will choose some great intellectuals to work for you. Niranjan did the same. He has made man like himself. That is why the human body is called as the Narayani Chola. Man has more awakening in him in comparison to other species. Further, it is your Sons who complete your left over work. Likewise, it is man whom Niranjan has chosen for the completion of his various works. If there is anyone who has decorated the world, it is man and man alone. He has constructed dams, palaces, roads and other countless things. The lions, dogs or the donkeys etc. can’t do such a job.
That is why he had to create human beings. He made him out of all the 5 material elements with him and gave him the needed Knowledge for the purpose. Seating himself within everyone as Mind, he makes all of us wander as per his sweet choice. (Regulating Gunas-Quality of thoughts) Though he is present in all the creatures, yet he can’t get the works done without them. It is only the human beings, who can execute these works.