I have no explanations as to why Muslims do anything, but just a technical note on Arabic grammar, contrary to the translations you’ve been given so far for this phrase:
The general pattern under which “akbar” falls is used to form superlatives (and I believe also other comparatives of degree, but it’s been a while since I studied this stuff in Arabic; I have dim memories of being taught sentences such as “akhi
akbar minni” for “my brother is
older than me”, but that might be colloquial), such that you will also find it for other adjectives. Take a root like H-s-n (ح س ن). It is not only the proper name “Hassan”, it also means “good”. To form its superlative form, you add an initial alif (long “a” sound) - ahsan ‘best’, as in a sentence like “lughati al-'arabi laistu ahsen” (my Arabic is not the best).
While I don’t know that this is followed strictly with the phrase under consideration (since it is highly conventionalized), following this pattern we can see that akbar is the superlative form of kbir ‘great/large’. So Allahu akbar would be more literally translated as ‘God is great
est’.
Now, I may be completely out to lunch with this one, but it seems to follow that given the general thrust of Islamic theology essentially being defensive and aggressive (defensive in that it is making dubious claims as to its continuation and perfection of previous messages and the religions based on them – i.e., Judaism and Christianity; aggressive in that it seeks to impose that self-understanding on others, even twisting and distorting their narratives and scriptures in order to do so), it makes perfect sense that Muslims would adopt this saying as an affirmation of the correctness of their religion before performing some extreme act in misguided service to it. After all, we all agree that God is great, but Islam alone – and within the historical context in which it has tried to graft itself onto previous Abrahamic religion and claim itself the corrective to all of them – claims that God is
greatest. In a very real way, “Allahu akbar” coming from an Islamist is a confrontational and provocative statement, because in it is an implicit rejection of other religions’ claims. Coupled with violence, “Allahu akbar” is, like their perversion of tawhid (monotheism), a weapon of Muslims against the preexisting Christians and Jews, and whoever else may claim to worship God. When they burn down your church while chanting “Allahu akbar”, they’re not only showing that they are stronger than you and can treat you as they please – they’re also saying that their God can beat up your God (to use an American phrase popular with bumper sticker philosophers).
Before you say I’m being unfair, have a listen to an Arabic-speaking Christian using essentially the same phrase in praise, not confrontation – here it is “Allah Kbir” (‘God is Great’; not the superlative form), an affirmation and not an attempt to one up others, and certainly not with violence:
Fairouz “Allah Kbeer”
Christians obviously agree that God is greatest, but have generally shied away from using that as a weapon against others. Not so with Muslims.