I'm guessing the people I know who do it are maybe not aware of the risks
There are a whole lot of people out there unaware of the risks.
One of the bacteria which can be found in powdered formula is salmonella, remember the big salmonella scare? (maybe not, I am likely much older than you) Parents are frequently wary of feeding a 6-12 month old a cooked egg, which in the UK are lion marked to show the hens have been vaccinated and there is a very very low risk of salmonella, and yet the same parents fed their new borns formula made with cold water.
There is a way which adheres to the guides and that is the way it was done about a generation back, make the bottles up (hot water that has only boiled once as previously said), rapid cool, then keep them in the bottom back of the fridge. The ideal is to make each bottle fresh as needed, but this method (to make up bottles in advance) is also listed as acceptable in the 'birth to 5' book new mums are issued with (ask HV for one) which I believe is the same as the WHO info (though not been to read it recently).
Depending how long you plan to be out it's possible to make up a feed and take the bottle out with you, like Laura said it's good for 2 hrs so you have a 2 hr window to the feed time and a further 2 or 3 (or later 4) hrs until the next feed is due by which time you may be home and able to make the next feed fresh. That's a pretty long time to be out. You can also carry bottles in a cool bag with a freezer block in it (available at the £1 shop) although that's added weight.
Another thought - feed straight from the fridge
Mine took it fine and many do. I chose to use pre-made formula for all feeds (no guilt here Happy, we can all spend our money where we choose hey
) and poured into a sterile bottle just before a feed was due. I used large cartons which are kept in the fridge. Depending where we were going I would either fill a sterile bottle up just before heading out or take a carton, a pair of clean scissors and an empty sterile bottle with me. DS drank it room temp if it was a new carton I'd just opened or fridge temp if it was a pre-opened carton stored in the fridge. it made no difference to him and totally avoid having to wait for something to get to room temp or to warm it with jugs of water or whatever when you are out. It really cuts down on the hassle whilst sticking to feeding guidelines and eliminates the difficulty of night time bottles needing to be brought up to room temp too.
oh posted, same time as a couple of others, I see Laura has replied about keeping bottles in the fridge too.