It is much fairer to use GDP.

If a nation chooses to have lots of social programs the government would need to have a higher income. This would result in higher NATO spending target. A nation that doesn't spend on social programmes would likely have a smaller government income and a reduced NATO commitment.

Additionally how the nations government works may have an effect. If a nation has lots of smaller local authorities that raise their own money and spend it themselves (like US states) then the federal/sovereign government would have a smaller income. An income that doesn't really match the nations governments spending.

GDP solves this and makes it agnostic to government structure and policies. The stronger your economy the bigger the commitment you make. Countries with less GDP per person tend to have lower labour costs. So manufacturing and military wages scale well relative to the countries population.

Probably because the 3%+ of the (very big) American economy might go away if trump gut gets elected.