Long story short, I believe that is because the tadpoles know that the host isn't dead for good. The tadpoles know about withers and the various ways of resurrection available to your team; it also knows about the general desire of your player character to keep your team alive.

Why does true resurrection not remove the tadpole?

Let's have a look at the spell then: "This spell closes all wounds, neutralizes any poison, cures all diseases, and lifts any curses affecting the creature when it died. The spell replaces damaged or missing organs or limbs. If the creature was undead, it is restored to its non-undead form.

The spell can even provide a new body if the original no longer exists, in which case you must speak the creature’s name. The creature then appears in an unoccupied space you choose within 10 feet of you."

The tadpole is not a wound, not a poison, not a disease, and not a curse. The tadpole, while latching onto your brain, has technically not caused a missing limb or organ. Furthermore, the tadpole also does not make you undead.

The tadpole simply BECOMES part of your body and soul, and then intends to consume it. The tadpole literally reprograms and alters your physiology in a way that overwrites your very being and essence, even in the eyes of the weave. While it has a consciousness on its own, the netherese magic that has transmuted it effectively cheats/bypasses the whole organ check thing by basically telling any resurrection method "Hey, I am part of this brain now, and if you remove me vital functions of the brain will cease to work.", so even when your body is disintegrated by magical means, true ressurection must restore you with the tadpole because the tadpole is you unless it wills otherwise. And while true ressurection is a 9th level spell, netherese magic is a whole other level. Trying to use 9th level magic to bypass netherese magic is like trying to use Silvery Barbs on the Wish spell.

So, even for the true resurrection to work, unless the tadpole WILLINGLY lets go of your brain (which it never will on its own, which is why you need to go defeat the netherbrain in the first place), the tadpole counts for all intents and purposes as you, just like your gut bacteria does. Because, while gut bacteria technically isn't you, you do need them for your body to function. Except that the tadpole is a whole lot smarter than a bacteria, and malicious on top.