"Why does matlab save strings from delimited text file as individual characters?"
It doesn't (unless you tell it to). How data is imported depends on the function that you are using and what options you used, for example functions like textscan can import string data as whole character arrays in a cell array, whereas readtable imports whole strings into a table. You do not tell us a single piece of information about how you are loading your data, so we have no idea what you are doing.
You are getting an individual character in your example because that is exactly what you are accessing with that indexing. If you tell MATLAB to extract one character then that is what you get.
Note that in order to perform numeric calculations on numeric data you will need to have numeric data. Your explanation tells us that you have the data in a character array. MATLAB is just doing what you tell it to do.
Most likely the simplest solution would be to import your data correctly: strings as strings, numeric as numeric. This will avoid the problems you are having entirely. If you upload a sample file then we could help you to load the data in a much more reliable way: edit your question, then click the paperclip button.
