The UV makes the polystyrene goes brittle.
I had an old Met one that I’d kept under my desk at work for maybe 2 years after upgrading it. Nowt wrong with it per se, I just *had* to have a Spesh Air Banshee. Anyway, we were moving offices, so as part of the clearing out process, I tossed the old lid into the bin from about 6′ away – a gentle throw for accuracy, NOT a full “lets destroy this” violent effort (think dropping it from waist height onto the floor). I kid you not, the lid shattered on touch down into 10 or 12 bits. I was really surprised how fragile it was.
I’ve been a bit more proactive about lid replacement since then…