

That’s not quite the same thing - the 7 digit phone number has more to do with short-term memory capacity than visual perception. Miller’s Law of short-term memory is we can store 7 +/- 2 items at a time, depending how complex they are.
https://en.m.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two
Because, I shit you not, it’s cheaper than adoption in the US.