"seem to". Christianity is a whole lot less homogenized than Islam, which itself does have distinctions.
The funny thing with Islam is that it is, AFAIK, less centralized than Christianity, whose primary denominations (at least by membership) tend to belong to larger governing bodies who control doctrine, ordination, such as the Catholic church, the Lutheran synods, the Anglican Communion, and the branches of Eastern Orthodox Christianity. Despite it's apparent schismatic nature, Christianity has a lot of central authority even if it is weak authority.
Islam as a religion doesn't have any central structure like this, excepting within Islamic states where it is controlled by the government or organized within a given nation. There's no central authority that ordains Imams or is able to establish an orthodoxy, This has been one of the major problems with Islamic Fundamentalism -- you can't go to the Islamic pope and say tone it down. Anybody can grow a beard, grab a Koran, claim to be an Imam and try to be more conservative than the guy next door who he labels an infidel.
Most so-called Christians aren't particularly religious,
And this is largely what makes Christianity in many ways more compatible with secular modernism -- the dominant flavors available don't lay outright claim to civil governance. Unlike Islamic upheavals, the 30 Years' War was settled with the Peace of Westphalia which established an idea of non-interference in other states' internal affairs, putting an end to the imposition of pan-state Catholic meddling in national affairs and in effect greatly curtailing theocratric governance in the Christian world.
Christian belief tends to be more personal and its edicts begin -- and end -- with its members, and they don't really have explicit civil authority. The Islamic world still is invested in notions of theocratic governance -- there isn't civil law and religious law, there is only one body of law, Sharia. I think one thing that frustrates Muslims is this lack of enshrinement of their religious beliefs in civil law.