What is the reason that no man has or ever will keep the commandments? The answer is spelled out in Romans 8:7 where scripture does say "IS NOT SUBJECT TO THE LAW OF GOD AND NEITHER INDEED CAN BE" - that is the REASON given by Scriptures why no man has or no man ever will.
We have been through this. When a man is "carnally minded" he cannot obey the law of God. But scripture shows men can think spiritually.
Mat 26:41 Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak.
Men are not always carnally minded. Here Jesus says the disciples were "willing" to be obedient in their spirit, but the flesh was weak.
Man is more than flesh, he is spirit, soul, and body. In his spirit man can be willing to be obedient. This is what Paul is describing in Romans chapter 7. Paul is speaking from the perspective of an unregenerate man because he says he is "sold under sin" and "captive to the law of sin". This describes an unsaved unregenerate man. He also never mentions the Spirit in this chapter, not once.
Yet, Paul repeatedly says it is his will to obey God in this chapter.
Rom 7:14 For we know that the law is spiritual: but I am carnal, sold under sin.
15 For that which I do I allow not: for what I would, that do I not; but what I hate, that do I.
16 If then I do that which I would not, I consent unto the law that it is good.
17 Now then it is no more I that do it, but sin that dwelleth in me.
18 For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
19 For the good that I would I do not: but the evil which I would not, that I do.
20 Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
21 I find then a law, that, when I would do good, evil is present with me.
22 For I delight in the law of God after the inward man:
23 But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members.
24 O wretched man that I am! who shall deliver me from the body of this death?
25 I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the flesh the law of sin.
Of course, you will never admit this is Paul speaking from the perspective of an unregenerate man because it absolutely refutes Calvinism.