You seem to be convinced that religion 'doesn't improve anybody's quality of life'...
I've yet to see any lasting improvement in anybody's life that is unambiguously the product of religious belief. Even if there is a pronounced relationship between life improvements and the adoption of religious beliefs, we don't know what caused the other. Did religiosity cause the improvement, or did the improvement cause the religiosity? We generally cannot tell if there is any causal relationship here, only a relationship. However, many Christians are quick to take credit for improved lives regardless of what caused what.
...but 80% of the world practices some form of religion. Why would such a significant portion of the global population practice something that is actively harmful to them?
I don't know. People smoke, drink, and abuse illegal drugs, too. The harmful effects of such practices don't keep people from engaging in that kind of activity. I have seen people turn a blind eye to religion's harmful effects very often. Maybe religion is like an addictive drug that maintains its use despite obvious harm.
Anyway, if you are arguing that a practice is good because a large number of people engage in it, then you are arguing a non sequitur fallacy. It does not follow that if many people do something, then that behavior is good.
This is a real question, not a gotcha. Your premises seem to need further examination.
I think I answered your question to my satisfaction if not yours. Until I actually see any good evidence that religion is harmless if not good, then you have not convinced me regarding what you claimed in the OP. You made those claims, so you have the burden of proof.
The argument isn't that religion is good because a lot of people practice it, the argument is that if so many people are actively practicing it, then those people are seeing some appeal to that practice that either you aren't seeing, or aren't admitting to. The point is that in their view religion is satisfying, your point of view has no bearing on their engagement or enjoyment of religion.
You're making the linear argument that everything humans practice has to somehow be linked to social good, or has to be productive. This isn't how people actually work in practice. They do things that, when appealing to strict logic, might make no sense, but they find them appealing and enjoyable nonetheless.
This is the line of reasoning that I see most atheists apply to religion - it hinders some ambiguous form of 'progress' that religious people are too ignorant to know they want. If 80% of the world's population are happy drinking, smoking, practicing religion - why should we stop them?