Ya know, Mary, I think some of them are just better than others. I'm thinking back to a kit I bought for do-it-yourself, custom length zip ties, and those things have lasted forever! I used them to hold a big broken pot together and they held till the pot disintegrated. And that was several years sitting outdoors year round.
On the other hand, there's that container of pretty colored ones DH bought for me. I tried using them to hold a bit of steel panel to a post out front. Ha! Broke after about 6 months!