Think of it from the government's perspective. All an IRA is, is a way to get out of paying taxes on a portion of your income. The government doesn't _really_ want to cut off its income stream any more than is necessary, so they put restrictions on it. The most obvious place to start is limiting how much you're allowed to shelter from taxes in the first place.
Framed another way, it's another form of progressive taxation. If you assume that people save for retirement in proportion to how much they earn, then setting a flat limit of how much of your retirement savings can be sheltered from taxes effectively means that less wealthy people can shelter all of it, and as you get richer you pay taxes on a higher percentage of it.
Once a worker has enough savings such that he won't be a public charge in retirement there's no public interest in further savings. We don't need to incentivize behavior that only creates private benefits, such behaviors are their own reward.
It's not indefinite. Traditional IRAs have mandatory withdrawals, and lifespans are finite, so there's a clear bound on how long the taxes could be deferred.
