@georgia It is necessary in any nation past a certain size, unless you institute a series of selection processes to purge the population of those who would rely on "man made text is subjective and imperfect" to excuse themselves and go against the nation.
I think you confuse strict adherence with belief for infallibility. I've never seen anyone claim any constitution is infallible, they just claim it is inarguable law under normal conditions, which they're right about within their borders. It can only be argued through constitutional courts, in the same way that a divine text is argued by theologians and ecumenical councils, not on the streets when it's convenient.
Plus as I said, if you treat religious scripture as wordly law, all you get are holy wars, because the followers of every religion are 100% sure it's their holy text that's infallible and the rest are infidels. It's either enforcement of wordly law in the physical world, or replacing countries with theocracies with only the official religion's adherents being allowed in (or the third option, perpetual jihad).