I started bottle-feeding after 6mo so I guess their tummy is less sensitive then.
As Laura said, there is a risk of bacteria in the powdered milk which must be killed with very hot water. One of those bacteria is salmonella, I don't think a tummy being less sensitive protects a baby from such bacteria. People are super cautious with cooking eggs but not so cautious with formula.
My personal feeling is that people became confused when the guides changed to making up bottles at each feed rather than the more old fashioned way of making up a full days bottles in the morning like a production line, rapidly cooling, then keeping them in the fridge (bottom back shelf). The trend now is to make up bottles with pre-boiled but cold water which is not correct as pps have said.
The guides mentioned that if you were going to be unable to make up a bottle at the time of the feed it was fine to pre-make, rapid cool, store cooled (fridge if it was going to be more than something like 45 mins or a cool bag with ice pack if out and about) and use that any time of the day as it lasts many hours that way. This method does mean the bottle is made in advance but is still safe. It means you can carry less stuff around with you.
If you use the ready made (I did) be prepared for waste. Either decant to 2 or 3 bottles for the day before heading out or take additional sterilized bottles with you to decant into on opening the carton. I've seen the newer bottles of ready made which look great. The large cartons of ready made work out a little cheaper, store in the fridge for I think it's 3 days. I used to pour a bottle just before heading out and about and serve room temp (or rather hand bag temp!). I also kept a few small cartons in the car so could take out just 1 or 2 sealed sterilized empty bottles if there was a risk of being delayed whilst out etc.
breast milk seemed so much easier than this!
Yes on the surface BF does seem so much easier in terms of the milk being perfectly ready and available but for people like me who struggled terribly to BF I became aware it is not physically easy for everyone. I was gutted when I had to get bottles, formula and all that but you get into the swing of things quite quickly.