Because it's not like Kisame doesn't have a Huge Shark bullet that can absorb chakra from ninjutsu,
Except Mokuton is solid, the time to absorb the chakra in it, he will already receive the damage of it, not even mentioning the large scale of mokuton we're talking about here
It's not like Itachi has amaterasu
He will burn bunch of branches and after that? He will collapse as usual because of the strain obviously which will leave him open
It's not like Pain can one shot flower Tree/wood dragon with shinra tensei, It's not like preta path can absorb chakra,
And here you underrate the scale of the Mokuton we're talking about here. Preta path is not absorbing that, and a small Shinra tensei can't destroy all that. Nagato would need huge Shinra tensei to deal with all the branches and after that there is a huge time limit where he couldn't use it anymore
It's not like deidara can't blow it up, It's not like kakuzu powerful elements of all 5 elements, It's not like Konan cannot assist with millions of paper bombs, Hidan could hack at it with his scythe (lol), it's not like they have any dojutsu users so they cannot see the spores.
It's not as if i've not shown Hashirama's tajuu Mokubunshin. He can send some of his clones to deal with Deidara, Kakuzu and the rest. His clones have shown great resilience and can perform powerful ninjutsus so they won't be that easy to deal with
How could they counter the equivalent of Perfect Susanoo(Wood god technique) that destroys a mountain in one move? The wood dragon as well that can bind Kyubi?
You're the one underrating Hashirama here. He would probably not be able to win, but he won't be put down that easily as well. And when you add his brother who made Sasuke and his group shit his pant just by raising his chakra, best suiton user in the manga and thus puting even Kisame to shame, possess S/T techniques, Edo tensei, etc, it won't be as easy as you think