Why do we saya atomic spectra are like fingerprints of the elements?

Because no element can produce exactly the same result in an atomic spectra.

atomic spectra is the range of frequencies of light emitted by atoms. { atomic spectra of light is unique spectra of light emitted by an element when electricity is run through it and when it is viewed through prism because they are unique they act as atoms fingerprint }

atomic fingerprint is a term which is used to define characteristics of given element and can be used for identification