Because whenever a verb's conjugation makes clear who the subject is, the pronoun can (and, usually should) be omitted. That's true in Italian and also in many other languages with similar conjugation rules.
Having "Io" won't make it strictly wrong, but it will sometimes sound a little odd.
In English, "I am doING" something, where you pair "to be" or "to have" with the gerund form of a verb, is called the present progressive tense. There is an analogous tense in Italian, where you pair the present indicative conjugation of "stare" with the gerund form, which in this case would be "mangiando", thus: Sto mangiando. Generally, it's used much much more in English than it is in Italian. For this reason, I think it's a bad habit to get into to mentally translate the italian presente indicativo into the english present progressive. So while "I am eating sugar" carries the same English meaning, you shouldn't translate it that way. "I eat sugar" is the correct translation.
I understand that Italian has a present progressive, but would an Italian speaker use it consistently, reserving the indicative for the equivalent form in English? My impression is that, idiomatically, Italians often use 'mangio' in this progressive sense, transitively or intransitively, as the French commonly do. Perhaps this isn't so ... but,if it is, I want to be aware of it, since the difference in intention between 'I am eating sugar' and 'I eat sugar' is quite significant.