Because fruit is a metaphor for only the positive entities. Fruit of womb, vine etc. There are no fruits of flesh or sin, its outcome is unfruitfulness, faith alone, lawlessness. So only the works of flesh is suitable for (akarpos) unfruitful people in Paul's context.
The bad fruits of the false prophets or believers are actually fake fruits, not a poor quality apple, but thorns & thistles. The fruits are the practical, spiritual result of their works. The nature of context in Matt 7 suits bad fruits rather than "unfruitful" as it talks about verification through fruits, although the meaning is the same.
ESV
Matthew 7:16-19: “You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire.”
ESV
Ephesians 5:9-11: “(for the fruit of light is found in all that is good and right and true), and try to discern what is pleasing to the Lord. Take no part in the unfruitful works of darkness, but instead expose them.”
Matthew 13:22: “As for what was sown among thorns, this is the one who hears the word, but the cares of the world and the deceitfulness of riches choke the word, and it proves unfruitful.”
Mark 4:19: “but the cares of the world and the deceitfulness of riches and the desires for other things enter in and choke the word, and it proves unfruitful.”
1 Corinthians 14:14: “For if I pray in a tongue, my spirit prays but my mind is unfruitful.”
Ephesians 5:11: “Take no part in the unfruitful works of darkness, but instead expose them.”
Titus 3:14: “And let our people learn to devote themselves to good works, so as to help cases of urgent need, and not be unfruitful.”