Because regeneration is defined as holiness. I'll clarify.
When we say that God is holy, note we don't find it necessary to add, "And as an added bonus, He is not the type of person who sins periodically."
That would be redundant. Holiness excludes sin. Since holiness defines regeneration, any sin would by definition constitute a deregeneration. You'd no longer be a child of God. The only reason that you find this argument surprising and confusing is that, in sheer intellectual dishonesty, evangelical theologians have been in denial about this contradiction for 500 years. They neither discuss it nor mention it. That's where I come in. I call attention to it, and then solve it by arguing that only a portion of your material soul was regenerated. Your sinful nature lurks in the remainder of your soul. Actually this is pivotal to Paul's argument in Galatians 3:1-6. There he is arguing what I said earlier - sanctification is the process of being incrementally filled with the Third Person (i.e sanctification proceeds by outpourings upon those parts of you still unregenerate).
P.S. If you weren't aware that holiness defines regeneration, I here provide you with one proof (but there's definitely more than one). On sixty occasions, the NT refers to the churches as "the saints", using the Greek adjective for holy and thus literally as "the holy ones". That's the same greek adjective used 90 times in the title "The Holy Breath".