A lot of people leave the bluetooth on but just not connected
Really? From what I've seen they turn it off as soon as they can to save battery life.
There's many accessories that rely on bluetooth nowadays: smartwatch/smartband, headset (moreover with the newest trend of ridding the 3.5 mm jack), etc.
I used to do that, but my new phone has better batteries, so that isn't an issue any more. Combine that with my bluetooth headphones, and it's more convenient to just keep BT on at all times.
It's easy to forget. I probably leave my phone's wifi hotspot on accidentally about half the time too.
A lot of people also forgo carrying a smartphone completely for privacy reasons
I never turn off BT on my phone, since I don't want to tinker around with it when I get into the car or turn on a wireless BT headset.