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Because the opposites of these attributes are deficiencies.
The opposites of knowledge, power and life are ignorance, incapacity and death. To attribute such defects to the independently necessary Creator would undermine the claim of divine perfection. Affirming perfections and denying deficiencies thus belong to one line of argument.
The reasoning must not be transferred mechanically to physical limitations. A person's inability to see may be a deficiency, but God's lack of dependence on a human eye is not a deficiency. We must distinguish the perfection of an attribute from the created organ through which a human experiences something similar.