The person driving the ECV is paying attention. But just as with larger/commercialish motor vehicles, the beeping is to alert everybody else in the area of the action.
We so know nobody pays attention to the ECV horns in general. Loud beeping may be annoying, but if some manufacturers didn't think it's necessary, they wouldn't add it.
To my knowledge, there are no
personal mobility devices that "beep" (when backing up) that are sold in the US. Not saying there are none - but aside from one model that I came across (when originally searching for my first personal mobility device) that was manufactured overseas, and shipped to the US for sale (and has since gone out of business in the US, apparently due to poor sales) I have never personally encountered a mobility device sold for personal use that beeps when backing up.
The first and only mobility devices to be equipped with "beepers" were the motorized store shopping carts. According to the company that claims to be the first manufacturer of a motorized store cart (which was essentially a modified standard shopping cart married to an early 1970s personal mobility device) they added the "beeper" because they thought it would make it safer; the "inventor" was used to working around industrial equipment that beeped when backing up, and so it made sense to him to add the beeper for a device being used in a commercial environment. (The first shopping cart was invented by a guy from my home state of Oklahoma - as was the parking meter - so we tend to be the guinea pigs around here for any new "innovation" in that space LOL) The jury is still out, literally and figuratively, on whether the beeping helps;
people still apparently regularly put them into reverse and back up without looking, believing that because it beeps, they don't have to look behind them - the beeping will alert others to their action.
So, my point remains the same - if you are operating the device in a safe manner, checking behind you, backing up carefully, there should be no need for it to "beep". And if the beeping really did help prevent issues, and made the devices "safer", then all motorized personal mobility devices would have beepers.
(Also, take a moment, and try to imagine WDW if every single mobility device - wheelchair or ECV - "beeped" when backing up? :::shudder::: It would be an auditory nightmare!)