So for example the capital O on license plates in California is only distinguished from the zero by being slightly more squarish, the capital G is mostly distinguished from six by six being slightly more smooth and diagonal in its top arc. I and one are a bit further visually, as are B and 8, but it would probably fool a traffic camera that was taking down plates automatically.
In addition, all-numbers-plates, I believe, are reserved by California exempt plates (emergency vehicles, police), and vanity plates are absolutely a thing, much more likely to start and/or end on a letter, so that's why you see numbers at the beginning and end. Like you can kinda see “6EIC023” and say “oh yeah my car looks like an ad for Geico” but because the start and end are numbers it doesn't occur to most people.
All-numbers would be worse not better? That's only 10^7, even if they kept the first one 1-9 and did 26^7,they'd have billions, seems like the obvious solution, but I take it there must be some limitations that make it hard to go there.
In addition, all-numbers-plates, I believe, are reserved by California exempt plates (emergency vehicles, police), and vanity plates are absolutely a thing, much more likely to start and/or end on a letter, so that's why you see numbers at the beginning and end. Like you can kinda see “6EIC023” and say “oh yeah my car looks like an ad for Geico” but because the start and end are numbers it doesn't occur to most people.