Why is it that people always talk about only one side of the accounting ledger?

The US has a net worth of nearly $150 trillion dollars. Who cares about the absolute value of debt?

That $150 trillion - is that book value, or fair value?

I assume it's a combination of both; I very much doubt that all ~$250 TRILLION in assets are being marked-to-market.

And, being as how both assets and liabilities will use the same accounting measure, I don't think it matters.

That's not absolute debt, that is debt relative to GDP as a percentage. Its a measure of, effectively, how long it would take the US to pay off its debt if it forced itself to operate in the green, since there is a limit on how much you can tax out of an economy without causing it to contract, and GDP is a flawed but relative measure of what you can potentially tax.

So where is the equivalent graph showing the rate of growth of assets as a percentage of GDP?

It's one sided accounting that people love to trot out as a big scary thing. The net worth of the US is growing; what else matters?

The federal government doesn't own the entire country. What's the net worth of the federal government?

It's a function that takes into account how much it can tax the entire country, of course.