Quote from atticus:
Exactly, it's simply illogical to do so. Of course Romney's wealth is a tiny minority. Hardly an issue, but the lack of transparency will bite him in the ass come November.
Not really illogical because the growth in the assets is tax-deferred, so you can buy and sell and never pay any taxes until you withdraw assets as income. For a lot of people, though probably not Romney, there is also the benefit of withdrawing assets and being taxed on them at a lower rate than during their working years.
There's also the issue of wanting (perhaps) to have a single point of control on your long-term investments, so rather than having them in multiple accounts, a single IRA account makes sense.