The calculation works only if each customer has their own fiber.
Unfortunately for most (consumer) FTTH deployments, that's not the case. Most of them are GPON, where the initial deployment was more effective, as up to 64 of your customers share single fiber, but then that means all of them have to upgrade all 64 of them at the same time, you cannot do them one at the time (see also the speed of XG-PON upgrades).
Additionally, many providers forced use of their CPEs. If you can send out SFP module and the customers can put it into whatever they want, it is much simpler, as replacing CPEs for all the customers on that fiber.
Unfortunately for most (consumer) FTTH deployments, that's not the case. Most of them are GPON, where the initial deployment was more effective, as up to 64 of your customers share single fiber, but then that means all of them have to upgrade all 64 of them at the same time, you cannot do them one at the time (see also the speed of XG-PON upgrades).
Additionally, many providers forced use of their CPEs. If you can send out SFP module and the customers can put it into whatever they want, it is much simpler, as replacing CPEs for all the customers on that fiber.