The single letter variables are opposite of confusing: they bring simplicity and clarity. This is not programming, where if an integer represents a destination address, you can meaningfully call the variable destination_address (or destinationAddress or dest_addr or whatever). In math, an integer is often just any integer, and rewriting formulas to replace "i" with "any_integer" will not make them any clearer.

If math notation is unclear to you, it's only because you spent little time learning and using it. Mathematicians care about clarity even more then you, as they actually do indeed spend large parts of their lives reading and writing math. They are very quick to adopt new notation, if it brings meaningful benefits over the old one -- for example, the commutative diagrams and category theory language is now commonplace in all fields of algebra and topology, because it is much easier to draw a diagram and claim it commutes rather than name all the maps involved and write down all the equalities.