The legalization argument has strong undertones of free-market, common-sense economics to it. Solutions where government gets out of the way are not typically favored by government.

To modify your point:

Better question: why does $_GOVERNMENT so frequently think the right answer to a problem is $_GOVERNMENT_ACTION?

Examples include: $_HUGE_SPENDING_PROGRAM, $_HUGE_SOCIAL_WELFARE_PROGRAM, taxpayer-funded bailout of $_INDUSTRY, massive new regulations for $_INDUSTRY, price controls on $_PRODUCT, etc.

Maybe it's simply the general distrust of the individual and free markets by those in government, combined with voters who believe what politicians promise regarding that same government.