Why?

If your instant background checks are not effective in preventing firearms sales to dangerous persons, why do you need the same kind of checks on ammunition purchases?   You are applying the same failed "solutions" to a similar "problem", but expecting better results?

And if your background checks are ineffective in preventing criminals from purchasing firearms, why do you suppose that the SAME checks on purchasing ammunition will be more effective?

Blumenthal and Wasserman Schultz Introduce Ammunition Background Check Act | Press Releases | United States Senator Richard Blumenthal:

Under current law, just as criminals, domestic abusers, and dangerously mentally ill individuals are prohibited from purchasing a firearm, they are also prohibited from purchasing ammunition. Unlike firearms, however, federal law does not require a background check to prevent the illegal purchase of bullets. The Ammunition Background Check Act of 2018 would close this loophole by requiring all buyers of ammunition to undergo an instant background check under the same FBI National Instant Background Check System (NICS), the same quick and easy process that applies to the purchase of firearms.

Referring to the previous statement:

Federal law does not require a background check to prevent the illegal purchase of bullets.

(By that, one assumes that the ignorant author of the article refers to "loaded ammunition", not the "bullets" themselves.  One would wish that the press would get their terminology correct to prevent misunderstandings.)

It's curious that a law is imposed to restrict the purchase of ammunition by the people who are restricted from purchasing firearms.  Do members of the Federal Government think that the laws they have already passed are likely to be disobeyed

This does not speak well to their confidence vis a vis the efficacy of their original law restricting access to firearms.

When I was a Sergeant in the Army, I was taught to "never give an order which you know will be not be obeyed".

Apparently, the Politicians who have chosen to join in this autocratic movement to ban 2nd Amendment Rights have not yet learned this lesson.   Ultimately, they may learn that it is much easier to restrict rights to own firearms... but quite a different problem to restrict rights to ammunition.

The Second Amendment has often