There are many notions and only a limited number of symbols. Symbols have different meanings in in different areas of mathematics, but in a computer algebra system a symbol can only be bound to one object at a time. I expect that `i is I`

was introduced because of compatibility with other packages, and because "i" gets used as a counting variable very often, so is rather unsuitable as a symbol representing sqrt(-1).

If you want to use `I(n)`

you can by setting `I=identity_matrix`

at the start of your session (or even in your startup file `.sage/init.sage`

). Just make sure to include that binding whenever you share code with other people who might not use it by default.