Well - but this is just it! Here is an example where literal understandings suffer a difficulty that deeper readings don’t. If we’re speaking of a literal tree, then indeed … why?
But if the tree of knowledge of good and evil (not to mention the tree of life - which shows up again in Revelation) represent significant things then one can begin to at least muse and reflect on why God would not just “remove it”. For example, yes - a good parent wouldn’t tempt their five year old child with chocolates while at the same time forbidding them from partaking; but, as the child comes of age, a good parent will learn to “let go” and not demand obedience in compulsary ways, in the hopes that as a child learns independence, they will hopefully learn to make their own good choices and honor their parents out of a sense of their own love and no longer because the parent compels. One might then ask, “why should a parent ever ‘let go’? Because wouldn’t that be just like placing unecessary temptations in front of their child?” But we can see in this case why such a thing is needed. One does not do their growing children any favors by trying to maintain a protective moral bubble around them all the way up into their adolescent and young-adult years. And those families that do attempt that - it almost never ends well. True love only exists when it is freely given (or given back) - it can never be coerced or compelled. I think God wants true love from us.
It isn’t a perfect analogy, of course, - we are never independent of God, and God is never really “letting us go” in the sense of abandoning us. But it may help us see how examples do exist of what the tree of knowledge could stand for. Other examples could be volunteered as well.