Those who are in Christ do not make a habit of sinning. Those who do habitually sin are not in Christ. It really is that simple.
If we are still sinners after accepting Christ and being reborn, then the entire gospel is a lie. We cannot be what we were prior to Christ once we've been redeemed in Christ.
Why would anyone insist they are a sinner who sins and a Christian?
Paul gave a pretty good reason in Romans.
Rom 7:14 For we know that the law is spiritual: but I am carnal, sold under sin.
Rom 7:15 For that which I do I allow not: for what I would, that do I not; but what I hate, that do I.
Rom 7:16 If then I do that which I would not, I consent unto the law that it is good.
Rom 7:17 Now then it is no more I that do it, but sin that dwelleth in me.
Rom 7:18 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.
Rom 7:19 For the good that I would I do not: but the evil which I would not, that I do.
Rom 7:20 Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.