I think it's because comments like that are over-simplified and don't accurately describe the world as it is across the distributions of people.
For a hypothetical "Worldwide Average Person," things are quite better today than at any point in history. There are a lot of people who were dirt poor yesterday and are now less dirt poor today. For a small number of very well-off people, things are exponentially better today than ever. But for a significant number of people who had it really good in, let's say the 1950s and 1960s, things are noticeably and measurably worse in all ways today. If you're one of these people, the "Well that average person over there in Indonesia is better off" argument isn't convincing.
For a hypothetical "Worldwide Average Person," things are quite better today than at any point in history. There are a lot of people who were dirt poor yesterday and are now less dirt poor today. For a small number of very well-off people, things are exponentially better today than ever. But for a significant number of people who had it really good in, let's say the 1950s and 1960s, things are noticeably and measurably worse in all ways today. If you're one of these people, the "Well that average person over there in Indonesia is better off" argument isn't convincing.