In most industries for expensive items, manufacturers devise warranties to run out before the product is broken. Making it longer has a relatively small benefit (consumers might put a little bit more confidence in the longevity of a product with a 25 year warranty than one with a 20 year warranty), and making it too long has a pretty high cost (a bunch of warranty claims).

Especially if the useful lifetime is not well known, the incentive is for manufacturers to underpromise in their warranties. All of this applied to solar panels sold 25 years ago, and 25 years was long enough to sell people in solar panels and a line of credit as something that would pay for itself. In that context, I think it would be surprising if the panels didn't last far longer than the warranty promised.

It's just the time the manufacturer is willing (or forced) to be held liable for certain defects to be caused by manufacturing errors instead of expected wear and material degradation.

Unless something is mission- or safety-critical, replacing it before it breaks is wasteful.