There are more people now than ever before. Using just the number is misleading which alienates people from caring about the issue and undermines the goal. Let's say that in 1950 1 in 10 people died from cancer. That's 250m people. If every year 250m died from cancer, and there was no change, that means with a current world population of 8b, the cancer death rate dropped from 10% to 3%. By looking at the raw number, it looks like nothing has changed, there has been no improvement. But looking at the percentage, we have cut cancer death rates by 70%. This is why ratios are important, it let's you measure the progress. Are we doing better or worse? Raw numbers don't tell you that.
The important part of the article is that modern slavery is rising, because the percentage of slaves is increasing, which tells me the problem is getting worse. But by telling me there are more slaves now than ever before, and a quick research session tells me that that's not the whole story, suddenly I may not think there is an issue at all. And now people don't care and the sensationalism that was used to get people to support resolutions and invest in change has had the opposite effect which means the problem will just continue to get worse.
Believe it or not, some uncles actually enjoy spending time with their nieces and nephews, and parents like to have a break from raising their children. It's amazing what can happen when your family is barely functional