Larger sites were sometimes OK, but a lot of sites (both small and large) eventually wanted to make money quickly (especially during the dot com bubble), and started showing ads, unaware of the fact that the sketchy ad networks of the time would show malicious ads.

Pop-up ads were ridiculously effective (high clickthrough rate) since they literally popped up above whatever you were doing and you had to do something (close it) to continue browsing. This was before adblockers and even popup blockers.