They do that because they teach this...
V.
This corruption of nature, during this life, does remain in those that are regenerated;[11] and although it be, through Christ, pardoned, and mortified; yet both itself, and all the motions thereof, are truly and properly sin.
[12]
ref: Westminster Confession of Faith, Article 6, Sec. 5
and this...
II. This sanctification is throughout, in the whole man;
[7] yet imperfect in this life, there abiding still some remnants of corruption in every part;[8] whence arises a continual and irreconcilable war, the flesh lusting against the Spirit, and the Spirit against the flesh.[9]
III.
In which war, although the remaining corruption, for a time, may much prevail;[10] yet, through the continual supply of strength from the sanctifying Spirit of Christ, the regenerate part does overcome;
[11] and so, the saints grow in grace,
[12] perfecting holiness in the fear of God.
[13]
Ref: Westminster Confession of Faith, Article 13, Sec. 2&3
Thus they have redefined "temptation," which is common to man (1Cor 10:13), into being an "indwelling sin nature" which FORCES the denial of genuine heart purity being a possibility.
Thus you can quote all the passages of the Bible which specifically speak of heart purity to these people and those passages mean nothing to them because the stronghold of "inborn sin" reigns in their minds.
That is what is going on folks.
This is why these people come up with foolish twisted strawman rhetoric like this...
They'll ignore the heart purity scriptures completely. They'll ignore the plain statements in the Bible about Christian's not sinning. Without a Biblical leg to stand on they resort to ad hominem attack via the invention of strawman positions which they then ridicule.
Jesus made this statement and he meant it...
Mat 7:21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
Mat 7:22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
Mat 7:23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
MANY! MANY! MANY!
The MAJORITY of people who profess Jesus as Lord are going to be condemned at the judgment to their shock and horror.
Luk 13:23 Then said one unto him, Lord,
are there few that be saved? And he said unto them,
Luk 13:24
Strive to enter in at the strait gate: for many, I say unto you, will seek to enter in, and shall not be able.
FEW are going to be saved.
Which will you be?