Moreover, zippers often fail at the most inconvenient times and that stands to reason, as zippers are involved at either the beginning or end of some more important process and the interruption to said process is often highly inconvenient.
Let me give you an anecdote (it's only but one of many nevertheless a notable one). Years ago, I worked alone in an office with three young women, one was my secretary and the other two were helpdesk assistants (I was always well behaved and it's something I'd never risk doing these days).
One day they all left the office together for lunch leaving me alone to continue working. During that hour I went to the toilet and my pants zipper broke necessitating a quick retreat to my office where I would attempt a temporary fix. Back there, the only practical and quick solution I could come up with was to take the very large paper stapler capable of stapling a half inch of paper and staple my fly back together - note, this was done with my pants still on as it was too risky to do otherwise.
Anyway, in the middle of the process the women all arrived back from lunch. Fortunately, they all broke out in hysterical laughter. I'd hate to think what would happen if that were to happen to me again today, I'd likely be charged with harassment.
That never happens with buttons, one may come off but in such circumstances that's hardly the drama of a broken zipper.
I'd add, it seems that from your experience with zippers that you've been unusually lucky (perhaps you can afford more expensive clothes than me).