Because usually you can't study long-term effects before releasing a drug, and even then it can take a long time for them to surface.
I took antihistamines basically throughout my 20s. My allergy specialist said there's no reason not to. I developed some other issues and wanted to stop taking antihistamines to see if that would help (or get a hint whether they were causing it) - but that got me into itching hell for weeks.
In some online forums people reported the same and shared ways to get out of that "addiction" without going insane from itching.
My doctor didn't want to believe it and there was no research on it. That only appeared a few years later in the form of a paper called "Unbearable Pruritus After Withdrawal of (Levo)cetirizine" (2016). In the US, the FDA issued a warning in 2025, which is also the time when I heard about it. None of the doctors I went to back then reported this to anyone, so I'm surprised it got discovered at all.
As for my other issues I have no way of knowing whether they had anything to do with long-term antihistamine use. I'm a sample size of one, and none of the other stuff is quite as clear-cut as "unbearable itching".
I've had other issues with prescription drugs that didn't make the official list of side-effects and sometimes those side-effects don't just go away once I stop taking the drug.
That's why I'm very cautious when trying drugs I never had before, and even more so when it comes to taking them long-term.