your carrier, the company you buy airtime from (ATT, Verizon, Sprint to name a few US ones) takes the firmware from LG and modify it the way they want it and then "Release" it on their system for you to download via system update. If they never put one out you will never get it. This happened to me a few times before on AT&T.
If you want the firmware LG release you can often download it from them directly but you may have to root your phone to actually install it.
Heres some: http
/g2-lg.com/lg-g2-firmware-stock-roms/