Galatians 5:17 English Standard Version (ESV) For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do.
When asked why God does not change our flesh, how do we answer? Not yet. Someday. When we are perfected. That does not answer why, just when.
Look at this from two points of view. The first is resistance training. Muscles get stronger when a load is place upon them. Weights are often used to help build muscle. Those weights must be applied appropriately so as not to tear a muscle. Training is required, skills developed and a conscious desire to exercise.
Flesh opposes the spirit and the spirit opposes the flesh. It is resistance training. Which grows stronger? If you walk in the spirit your ability to walk grows stronger. Walk in the flesh and the flesh grows stronger. As our spiritual walk develops our flesh grows more resistant. It is like adding weights.
The second issue is choice. A conscious decision on our part to love the Lord more than loving the flesh is required to train our souls and develop the skills to make sure that the flesh is subject to our desire to love God. This is one reason encouraged us to fast when faced with strong resistance.
Matthew 6:17-18 English Standard Version
17 But when you fast, anoint your head and wash your face, 18 that your fasting may not be seen by others but by your Father who is in secret. And your Father who sees in secret will reward you.
Fasting shows the flesh who is stronger and that is the soul that walks in the spirit.