They has been an accepted third person singular for close to a thousand years.

"It" is generally seen as appropriate for objects; refering to a person as such is generally seen as deeming them "not a person". Granted how far people are comfortable with extending this can vary. Some people get bothered by referring to close pets as "it", where as it isn't uncommon to see very young human children referred to as "it" in literature or conversation.

In regular conversation "he or she" is cumbersome even if it is correct by traditional rules, so "they" is often used. Likewise, "they" has been used frequently if one wants to reveal as little about the person as possible. While there are arguments for just coining a new term to add to the lexicon that plays by the traditional rules… these efforts usually get far more backlash than just continuing to use "they" in that way.