Passport Issue Date - warning!

Perhaps this should be in the "Worst Start to a Holiday" thread. But on Sunday the 28th of September, my best friend and I travelled to Gatwick Airport for the start of long-planned 10 day, all-inclusive holiday in Kos, Greece. I have been aware for a long time, that you must have 6 months remaining on your passport before expiration. My passport expires in May of next year (2026), so 8 months left! When we went to check in, my friend had no problem at all, and then I handed over my passport to the BA attendant. She looked at it and told me I could not fly!!! I was mortified and she explained that even though it did not expire for 8 months, the ISSUE date was actually 17 September 2015 and as that was over ten years ago, I was not allowed to fly!! My friend and I panicked a bit, then sat down and had a drink (her, a pint to start her holiday, me - coffee as I knew I needed to use my brain.) Only by the grace of God, was I able to travel the next day and join my friend in Greece. I became a British citizen in 2006 (originally from USA) and obtained a UK passport which I have travelled on ever since (renewing, obviously prior to 2016). After the pandemic when travel restrictions eased up, I booked a trip to the US to see my dear grandchildren! I found out, that the US at that time (summer 2021) was only letting American passport holders in, so I got an appointment at the US embassy in London and obtained a US passport. Back to the current trip to Kos, I got myself back home, picked up the US passport, had to book a new return flight at my own expense, which fortunately was not as expensive as I dreaded it might be, and I got to Kos to join my friend the next day.
Sorry for the long post, but neither my friend nor I, nor any of our acquaintances, knew of this "ten year issue date" restriction (apparently thanks to Brexit). I just want to make others aware of it to avoid a nasty surprise at the airport like I had. If I hadn't had the US passport at home, I suppose I could have tried to get an emergency UK passport, but it would have taken at least a few days, extra expense, etc. My final question, is "when you renew your passport, WHY do they add all those extra months on at the end,?" which creates this problem in the first place!!??