Why does the speaker think the nightingale is immortal in the seventh stanza of "Ode to a Nightingale"?

The poet does not really believe the bird he hears is
immortal. He is using what is termed "poetic license." Poetry does not have to be
literally true. This is also called a "poetic conceit." He is only pretending to think
this particular bird is immortal. Here is what suggests this "poetic
conceit":

Thou wast not born for death, immortal
Bird!
No hungry generations tread thee
down;
The voice I hear this passing night was
heard
In ancient days by emperor and
clown:
Perhaps the self-same song that found a
path
Through the sad heart of Ruth, when, sick for
home,
She stood in tears amid the alien
corn;
The same that oft-times
hath
Charm'd magic casements, opening on the
foam
Of perilous seas, in faery lands
forlorn.

The reason Keats can pretend the bird
is immortal is the fact that the nightingale, like most song birds, always sings the
same song. Robert Browning comments on this truth in his poem "Home-thoughts, From
Abroad."

That's the wise thrush: he sings each song twice
over:
Lest you should think he never could
re-capture
The first fine careless
rapture!

Keats was not concerned about literal
reality. He enjoyed using his mind for indulgence and escape, as he tells us in a poem
titled "Fancy" which begins with these
lines:

Ever let the Fancy
roam,
Pleasure never is at
home:
At a touch sweet Pleasure
melteth,
Like to bubbles when rain
pelteth;
Then let winged Fancy
wander
Through the thought still spread beyond
her:
Open wide the mind's
cage-door,
She'll dart forth, and cloudward
soar.

Members of the nightingale's
species have undoubtedly been singing the same notes for ages, perhaps ever since the
species first existed thousands of years ago. That is what leads Keats to think back in
time as far as he is able to travel in his imagination, drawing mainly upon his reading.
This takes him back to ancient Roman times, and then further back to biblical times, and
finally to times even older than the Bible, or to times that only exist in fairy tales,
which often begin with the words "Once upon a time." The nightingale who is allegedly
singing to the poet as he is composing his ode is thus the same nightingale who sang to
Ruth in the Old Testament. But this is not literal truth; it is a poetic conceit. It is
something Keats would like to believe--and he can believe it if he wants to because it
is his poem and his imaginary world. He can use words to describe anything he can
imagine. That, in fact, was the essence of Romanticism, wasn't
it?

This, of course, is not very practical. But poets are
not usually very practical people. Otherwise they wouldn't be
poets.