Because in my experience the s/s tanks gets growth inside in four to eight weeks and so are the plastic or nylon ones pretty soon while my epoxy or grp tanks last for a year without any substantial growth in them.
The rough surface of the s/s tanks and something in most older plastic tanks are a hotbed for growth of different sorts.

I filled the tanks and went to Europe for three months, came back and drunk all the water from the tanks. All was fine. Inside was no growth just very very little slime on the walls.
At the next fill up l emptied the tanks completely, flushed the inside with a bit of bicarbonate of soda and Bob's you uncle.
I don't even employ a carbon filter just a strainer.

Never ever used any chemical or soap or any other cleaning agent but bicarbonate of soda cleaning epoxy or grp tanks.

I suspect if you use food quality hoses that sunlight can not penetrate then there would be very little difference between SS and fibreglass.

I carry 1000 litres in the bilge divided into two tanks.
The floor has been glassed over and there are no inspection ports just the two filler caps which are under the gally sink which is a real pain to fill.
The gally tap runs through a puretap filter system which provides crystal clean drinking water.
showering every day , dishes and 4 loads of washing will last me close to 4 weeks before having to press the tanks up again.