Why do the bosses/raids in this game feel so lifeless?

[Vidu]

Why is there 2 Coeurl O’Nine Tails guarding magiteck terminals in the Thousand Maws of Toto-Rak? Aren't they plants who need sunlight?

Considering that they're carnivorous plants, they probably dont need a lot of sunlight, because they can get their energy from the little animals they eat. Also: If they do need light, wouldnt it make a lot of sense for them to hang around the terminals that emit a certain amount of light, even though they can survive without it (as Rafflesia, the T6-boss, proves - kinda the same species, but according to the lorebook it was burried under soil for quite a while before the Calamity)?

Why is there a Great Yellow Pelican, Deep Jungle Coeurl in Brayflox's Longstop? Are they guarding the Dragon? If so, why does the Dragon just eat the Hellbender in the middle of the fight?

...why wouldnt there be jungle creatures in the jungle?
The dragon is actually in Brayflox to heal its wounds, but I doubt that its in need of employing Coeurls and Pelikans as bodyguards, they're just wildlife that live in that area (look at all the pelicans living in eastern LN).
On eating the Hellbender: The dragon got hungry? Or maybe just annoyed by all the noise those people fighting that thing were making, so he got rid off part of the noise?

In Dzemael Darkhold, an army of Voidsent have been sent into the realm, conveniently with 3 random bosses that just sit around far apart from each other waiting for adventures to attack them.

Well, voidsent arent really the most friendly creatures - so I dont see anything weird about each of them setting up shop in a specific area of the Darkhold, out of the way of the others. After all, they're trying to make a living there as far as I recall - and thats most likely easier when each of them takes a certain part of the dungeon instead of fighting all the time...

In The Wanderer's Palace, there's a Goobbue and a Flan guarding it as bosses, despite it being a Nymian Palace taking over by Tonberry. Are Tonberry allied with the flan or something?

Actually, the Tonberries are citizin of Nym, who have fallen ill - or rather have been cursed. The large amount of Tonberries are sleeping within the Palace, while some research (for example in the Scholar-questlines in HW and SB) are trying to find a cure. They didnt take over the Palace at all - those Tonberries that we fight havent only lost their apperance (Nymians were lalafell), but also their minds. So, with the Citizin of Nym sleeping, wildlife, including Gobbues, who are, again indigenous to this part of Eorzea (we're seeing them only a stone throw away in eastern LN), have taking over the palace. The Flan is a low ranking voidsent, not 100% sure about his buisness there though...

Snowcloak is Ysayle's hideout, yet it's guarded by an Elemental, an Abominable Snowman and Fenrir. Like why? Did the heretics form an alliance with the Yeti or something? Do the wildlife of Coerthas support the heretic cause?
That's just up to level 50, and I could go on.

Well, if I were to hide somewhere, hiding behind a bunch of strong beasts seems like a good idea to me!
The wildlife doesnt need to actively support the heretics cause, they dont need an alliance - all they's need would be a secret passage (or their teleport) to get past the beasts that we have to slaughter. The wildlife can just go about its life without even knowing that the herectis are using it as a shield against any possible intruders.
Fenrir might have been tamed - seeing how we have one as mount and how Fenrir (or rather: hoarhounds, seeing how Fenrir is the name of one of them) pups are being trained/used in the Wolves Den, according to the minion description.


...I'm not saying that there arent weird choices of bosses throughout the game - but the bosses that resemble the actual wildlife of that area actual make a lot of sense to me. But I dont think of them as "guarding" anything, I just think of them living there, minding their own buisness, until we stumble into their living room and start throwing fireballs at them...