First, there's a huge difference between "strong" immune responses and "good" immune responses.

Triggering the adaptive immune system is dangerous, like unleashing Skynet robots against a zombie apocalypse. You don't nuke a city at the first police report of one person biting another. Your body has a ton of cascading safety interlocks to try to avoid triggering more than is absolutely necessary.

Second, there's a difference between "we need dangerous exposure to actual pathogens" versus "we need calibration against a mileu of benign species we co-evolved with."

There's no evidence our immune systems are somehow "weaker" than our ancestors', but they do seem to be miscalibrated and trigger-happy.