If there is never a drawback to reporting, why not report every game?

I see Trump slogans, I downvote.

On a serious note, if everyone did that then it will screw up the data for the IFS, and they would have to bring back report weights, which means they have to spend resources away from other features such as honor system / report cards.

Go do something else.

If you constantly do this, the next time where you reported an actual feeder who went 0/39/2 in teamfights, the system will just typically see it as "False Report" since you have been feeding them bullshit from the start.

Even if we assume reports do not lose value anymore, and even if that also means that report spammers don't get their reports automatically discarded anymore, there's still a drawback: It creates more noise for the system to handle and thus inevitably has a negative impact on specificity and sensitivity of the system. Meaning: More innocents get punished (even though that number is abysmal to begin with) and more actual offenders get away.

So, honestly, it's the exact opposite of what you say: It only has drawbacks to feed the system with false reports. And as somebody else pointed out: If false reports were to become a problem, Riot would most likely simply reimplement report weight in some form.