There is a nasty bacteria that can potentially live in formula powder because it's not sterile, cronobacter. It's dangerous and can cause an all manor of problems, that's why we have to sterilise the powder ourselves with boiling water.
The sterilising of the bottle is not really to do with cronobacter, but actually to do with the bad bacteria all around us in the air. When making bottles, prepping bottles, the bad bacteria can come into contact with the milk and because it's got sugars and is a nice warm temp, it can multiply. Formula breeds bacteria.
If we wash the bottles but fail to remove all the milk residue (even if it's invisible!) these bacteria remain present and can be mixed into the new milk and they can cause illness like gastroenteritis. If we wash the bottles but fail to remove all the milk, yet sterilise, these high temperatures kill any nasty bacteria we've missed. And even though some people choose to stop sterilising at 6m because 'baby's put everything in their mouth' this is a very poor argument as a baby's immune system is not 100% effective until way after their 1st birthday.
To explain in layman's terms: if you wouldn't let your baby drink old formula milk...then keep sterilising!