Frank Erskine wrote:

Thirty or forty years ago rubber bands seemed to last for ever - nowadays
it would appear that they're only intended for very short-term use.

I doubt there is much difference in the way that elastic bands are
made. Perhaps they were made of natural resin originally but whatever
they are made of these days will be the cheapest available suitable
material as suits throw away items that they are and have always been.

I think the OP's problem is that his bands are now in contact with
something that perishes them, chlorine or oil based most likely.