They are. What people neglect to mention is that some investments, houses included, fail to pan out, and leave you worse off than you were before. Simply assuming that a mortgage builds useful equity is financially illiterate: If houses were a sure thing, everyone and every institution would buy them.
The really insidious idea is the idea that buying a house is what a mature, responsible person does, and that you can't be a mature, responsible person unless you're paying down a mortgage. This is marginally less potentially harmful to your future happiness than the idea that mature, responsible people smoke five packs of cigarettes a day.