There is also abrogation in Islam, which means the "peaceful" verses in Islam were replaced by the more intolerant ones
A view created around 2 centuries after Muhammed by al-Shafi'i and by no means universally accepted.
Because the Qur'an is the word of God and that cannot be reformed or changed.
Although interpretation of it has changed considerably.
Now many of these interpretations are indeed contrary to common decency, the idea that Islamic teachings are reified and immutable is not borne out by the historical evolution of the religion.
Much of this problem is caused by certain Muslims' insistence that their faith has remained unchanged and their inability to accept that much of Islamic orthodoxy emerged centuries after the death of Muhammed based on hadith and legal judgements that emerged from the realities of medieval empires rather than the 7th C.
It's fair to criticise the aspects of certain Islamic teachings that do not reflect modern values, but to say that Islam must necessarily be set in stone, whereas other religions like Zoroastrianism, Judaism and Christianity are free to evolve ignores the significant evolution (and, unfortunately, devolution) that Islam has also undergone and continues to do so.