Because Jarls are kings. and again, Ulfric woul dhave the final say, like Istlod. I am not saying they had to call the moot, just that the immediate acceptance of the WGC irked the Jarls because their opinions were not sought. Balgruuf says this himself, nobody asked the Jarls whether they fully agree with the White-Gold Concordant.
Jarls aren't exactly Kings, a Jarl is a Chief. The Jarls didn't have be asked, they're minor nobles in the large scale of the Empire. They were told by their King, whom they all swore loyalty to. They didn't have to like it, nobody liked it.
I disagree that the Empire had no choice but to accept the Terms of the WGC as they were, but I also disagree with the idea that Skyrim SHOULD be Independent, and feel that the rebels are haughty/full of themselves.
We're not even fully aware of what the entire terms of the treaty are. It is highly unlikely four years of warfare and several months of peace talks came to the conclusion of a grand treaty with three lines. The Empire wasn't in a position to change the major three which are disliked. The Blades were all but destroyed during the years prior and during the war. Southern Hammerfell was already occupied by Dominion forces and the Banning of Talos was always going to be their demand.
The Empire wasn't in a good position at the end of the war. They had lost three entire Legions and the remaining Legions were under half strength. The Empire was weaker than the Thalmor prior to the war, the Empire was even weaker after the war. The Thalmor had two armies engaged in the war. It took three armies of the Empire to defeat one of theirs. The Empire at the end of the war had no idea what the Thalmor had, hell the Blades didn't even realize the Thalmor could do the Great War.
You say the Empire didn't have to accept the treaty? What makes you even say that? The Empire didn't even have enough soldiers to restore order in Cyrodiil, Skyrim and High Rock. Cyrodiil erupted into violence as criminal gangs took advantage, High Rock lost the city of Wayrest, Skyrim lost the Reach for two years.
Had the Empire rejected the White-Gold Concordat, do you really think the Empire would still be there? With their petty number of Legions remaining at half strength, they would still have to split their forces to restore order in the provinces but also keep Legions engaged on the border as the Dominion did their border skirmishes like the Second Era. The Empire would collapse, from the inside.
The Empire would still have to send soldiers to Hammerfell to assist the Redguards, they would still need soldiers in Cyrodiil to maintain order and they would also have to keep the borders protected in the event the Thalmor managed to muster another army. They still had untouched provinces full of Khajiit and Bosmer soldiers.
If I was the Thalmor, I would use Valenwood and Elsweyr as staging points to conduct raids on villages and cities. Legion can't protect all of Cyrodiil, watch the border, fight in Hammerfell and restore order in Skyrim & High Rock. The price of common goods would raise, bandits would raid the farms for supplies. Populations would turn desperate and there would be mass riots.
The Empire would simply crumble due to loss of Imperial authority. The provinces would take advantage. During the Civil War in Skyrim, that petty little war. We can take a look at the issues they are facing, now we'll times that by thousands in all provinces.
Proventus:
"My lord, since Ulfric Stormcloak's uprising, the cost of goods has doubled. I'll need more coin if I'm to provision us properly."
Balgruuf:
"Our coffers are nearly empty. I can scarcely afford to pay the guards, and we need every one of them in these troubled times. You'll have to make do with what we have, Proventus."
Olfina:
"Did you raise your prices, Carlotta?"
Carlotta:
"I had to. The farmers are charging me double for the produce I sell."
Olfina:
"Let me guess, they're saying it's because of this trouble with the Stormcloaks?"
Carlotta:
"Exactly. Road patrols were cut in half because the Jarl needed soldiers. Now the bandits are bolder than ever. They've been raiding the farms for food, and supplies are scarcer than ever."