One thing that you have gotten wrong is:
Greyjoys are not pledging allegiance to Daenerys. Their plan is be her ally and make Euron Greyjoy her King Consort, thus becoming equals of the Royal dynasty and take over all the seven kingdoms. (Yara/Asha and Theon are dissidents and exiles, they are not legal representatives of Iron Islands and Greyjoys. Kingsmoot chose Euron as Iron King legally).
There is no support for Daenerys in Westeros among nobles generally because People mistrust her as Mad King's daughter and she left Westeros as a babe and thus has no allies. We saw this when Qartheen merchant Prince asked Daenerys if she has any allies in Westeros? And she replies that she left as a child. To that the Merchant Prince replied that means she has no allies.
Except Dorne. Martells are conspiring to get the Targaryens back but
even they harbor doubts about her sanity (In books, in show they are wiped out)
We have seen this before that lack of contact with the nobles can deprive you of allies. King Aegon III's daughters were imprisoned by their Brother King Baelor I. They spent more than a decade locked in a tower. When their brother died childless, Princess Daena was heir of her brother but due to her imprisonment, she had no noble allies and crown passed to her uncle King Viserys II Targaryen.
Why should Robb Stark pledge allegiance to a girl lost in wilderness of the East when he has the chance to reclaim the Kingdom of his forefathers? Starks were Kings in the North for thousands of years until Aegon the Conqueror made King Torrhen Stark kneel and swear fealty to him. You answer yourself in the quote you cited:
MY LORDS! Here is what I say to these two kings! Renly Baratheon is
nothing to me, nor Stannis neither. Why should they rule over me and
mine, from some flowery seat in Highgarden or Dorne? What do they
know of the Wall or the wolfswood or the barrows of the First Men?
Even their gods are wrong. The Others take the Lannisters too, I’ve
had a bellyful of them. Why shouldn’t we rule ourselves again? It
was the dragons we married, and the dragons are all dead! There sits
the only king I mean to bow my knee to, m’lords. The King in the
He is not talking about Metaphorical dragons, he is talking about real dragons. It was Vhagar, Meraxes and Balerion who made the Northmen join the realm of Aegon the Conqueror. Now all the dragons were dead and dragonriders were almost gone from the earth. Why should they be ruled by some Southern lordling any longer?
That does not mean Robb never considered allying with anyone in War of the Five Kings. He considered only the people with real power in vicinity as potential allies, Daenerys did not fit the bill. That is why he laments that he could have married Sansa to a Tyrell and allied himself with High Garden. He also tried to make an alliance with Renly Baratheon even though Robb refused to consider him King. He also tried to forge an alliance with Greyjoys for mutual assistance in both their independence.
The Lords of the realm gave their verdict on claim of Viserys and Daenerys when they crowned Robert as King and sent Stannis to dragonstone to arrest the deposed Princelings. Tyrells did fight for the Targaryens till the end but now they are allied to Baratheons due to marriage between Tommen and Margery.
Dorne did try to rise up for Viserys but Jon Arryn's diplomacy
prevented that. (In books, show never depicted that era)
TL;DR, there is no incentive for the Nobles in declaring for Daenerys while the risk is enormous. Which is why even the most sympathetic nobles wouldn't rise for Daenerys unless she lands with her own power. The only people who are finding her and pledging fealty to her are the ones who have no other/better choice e.g. Tyrion, Jorah, Barristan, Greyjoy Siblings (In show) etc.
Season Six Finale Spoilers:
Dorne and The Reach have declared for Daenerys and joined her fleet
sailing to Westeros. Fire and Blood is coming. Bear in mind, during
Robert's Rebellion, Dorne and Reach fought till the very end for King
Aerys. Though the reason they gave for their declaration is that they
want vengeance for murdered Tyrells, Elia and Oberyn.