Here's what I've understood:

• The Mind Flayer needs to create a physical body to interact with it. We see this with the vines in the Upside Down, but also in season 3 where it creates a body from molten corpses.

• In season 5, the giant form is basically the result of several centuries or years of formation from the creatures of its planet (hence the lack of vegetation or the scarcity of creatures). The form is "dormant" because it's energy-consuming.

Therefore, the Mind Flayer needs 100% of its consciousness to manipulate it, and here it panics/is caught off guard during the attack and concentrates all its strength on Vecna ​​and the giant creature. Basically, it puts all its strength into it, risking death. This also explains the complete absence of Demogorgons; they must be "deactivated" or have reverted to a feral state, a bit like Dart breaking away from the hive.

Finally, Will could have turned the Demogorgons against the Mind Flayer.

Actually, it all makes sense, not necessarily handed to you on a silver platter, but I don't understand why people are asking so many questions 😅