What is the point of making him reincarnate as a baby just to have him grow up lmaoo Like ...

Him being born from the sacred tree is a very important plot point. That plot point is why.

I still don't understand the need to make him a baby at first then turn it almost immediately into a child/pre-teen.

Couldn't he just have been born already grown up? Couldn't they have let him grow up by day instead of by years? (Even that would have been way less weirder-)

I'm not mad or anything, genuinely asking ahah
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Let me try a different approach. When it comes to isekai specifically, all these stories fall under 2 main categories for isekai travel: Relife and Relocation.
What I'm going to focus on here is relife as there are many different subcategories with their own rule:

1. Rebirth - This is the relife system that forbids recreating your old body because it goes against the natural order and any relifed soul must start over in a brand new body.

2. Revival: this is the relife system that simply brings the target back to life in their original body before they are isekai'd. This typically requires a rare set of circumstances.

3. Recreation: this is essentially #2 without the original body.

4. Reincarnation: You remember your past life. (The "oh, I don't feel so good and now I suddenly remember my past life cliche) simple as that.

5. Possession: You are shoved in the body of someone or something that is not your reincarnation and you're forced to live as them. (Typically after they have died.)

6.Replacement: A very rare system is where you or just your soul is replaced by a fake and the fake is killed after you isekai. Basically, faking your death and taking you to another world.

So to answer your question, it's because this is rebirth, not revival. He's not in his original body. He can not be recreated in the form of his original body. It is not rebirth if he is not born as a baby. That is creation, not birth. He had to be specifically BORN from the tree of life.

If his body had been created aged up, then there is a chance that they would have mistaken him for an intruder. He could have been exiled or killed on sight.