Well actually no, some agents do offer 15 months. On the expiry of my initial 90-day Non-O back in 2021, the ThaiVisa forum agent here had two options for me, 12 months and 15 months, the latter being very much more expensive. I don't know how they would have got 15 months and the legal basis for that because I didn't go for that option, but they did have a price for it.
EDIT:
Here we go from my email correspondence back then:
The cost for a Retirement Visa for 15 Months is at THB 38,000 and Retirement Visa for 12 Months the price is at THB 26,000.