I kind of brought this up in another comment, that "first" and "1st" aren't really the same thing. Which is confusing when you extend that to fourth/4th five/5th. I don't generally see someone write "zeroith", but I'll see "0th".
First and 1st are certainly different symbols for the same concept
The spelling for the index before the first is zeroth, no need to insert an extra vowel
And here I thought people write "1st" because they are lazy and want to press 3 keys instead of 5.