Why do you think that is? Perhaps because it is Luther's opinion not supported by scripture. It isn't mentioned in any of the doctrinal statements for that reason. It isn't in Luther's Catechism.
Now are Catholics 'very quiet' about Pope Francis' support and exhortation of socialist principles, global warming, moral equivalence of Catholics and Muslims? Wait, isn't that last one in the Catholic Catechism that PF had nothing to do with? I guess Catholics are being 'very quiet' on Catholic doctrinal issues as well.
Luther said alot of things, as all Christians do, not all of them conform to what the Bible teaches. Those still living should be striving to conform themselves to God's Word, not our own desires and thoughts, no matter how pious we believe them to be.
I’ve made my point.
Peace,
Rich