Becase of the fear that most of them would spend it, instead of investing, and then they would have to go on welfare when they retire.
1. So even the Republicans really don't trust people with their own money, after all, despite the rhetoric.
2. They fear people would "have to go on welfare" presumes that welfare is good and necessary as well. Many think that the government shouldn't be in the retirement plan business (SS), or in the wealth redistribution business either (welfare). Something about the government being restricted to protecting "life, liberty and property." Oh well, the touching faith in limited government and enumerated powers.