well when you know what your doing you try to keep your ISO as low as possible.
sure most consumers and soccer moms will not keep such a close eye on the ISO setting.
in my small home studio i always shoot at ISO 100.
i have plenty of light so why should i go higher.
when doing nature and wildlife i try too keep the ISO as low as possible.
i never use auto ISO.
only time when i use auto ISO is is when i do city traveling.
then light changes often so fast, inside outside, shadows of buildings etc. that auto ISO is a help.
Most shooters don't have studios - whether its in the home or not. Most shooters are shooting where they are shooting, if that happens to be when the sun is down, then your using a higher ISO.
As a wedding shooter, the day starts early. Sometimes it's in a salon where yeah, there's big windows and generally lots of light, so ISO's are under 1000. Then where do you go, either a hotel room or home, which will have a variety of light. do you have the time to set up lighting? Sometimes yes, sometimes no, you just have to work with what you got.
what's next? Off to the church. some churches are filled with light and yeah you can shoot in the 1600 range. But, if it's not a sunny day, then all that glorious light from the windows is lessened. Then you have the darker churches, or the ones with a big skylight but not many other windows. For ceremonies, ISO can vary wildly!!!!
Oh, then it's time for those fun family shots which most want done inside the church. Shooting groups you can't exactly be using f2.8, your bumping that aperture up to at leastr 5.6, more preferably f8.
then it's the fun stuff, and yeah for most of the bride and groom shots your using closer to base ISO. But for the bridal party, you may want f8 or higher so even in daylight you may be as high as ISO 1600.
then it's to the reception, and yeah, lighting can vary wildly there too. high ISO and off camera light, or just high iso depending on the scene your shooting is the name of the game.
To say most shoot at base ISO is just really a narrow view of what, when and where people are shooting....