That is because we think faster than we type. Unless we type very, very fast ...
When typing informally I sometimes say s/he. That saves time. In formal communication the correct form is 'he or she'. In informal communication you can get away with 'they', but it is a bit awkward because then the verb won't be right. If in doubt, use the formal version.
The formal version of this in British English is to say, 'How do you do?'. The correct reply to this is, 'How do you do?' It is not intended as an actual question but a way of beginning a conversation; what is called by linguists phatic communication; a kind of social convention of how to begin. It works to let each side know that they are dealing with someone who understands the conventions of conversation, if you like.
This has changed into a whole series of different forms; some people say 'Hello' (best reply in return; 'Hello'), some say, 'How are you?' (best reply; 'Fine thanks. How about you?' 'Fine'. Saying 'Good' will identify the respondent as speaking American rather than British English, because good is a quality of character, not of health. In British English 'I am good' really means something along the lines of I am honest rather than I am well.
If you are asked this question by a total stranger the answer is always 'Fine'. If asked by a close friend you can go into more detail. Gradations in between will fall within this spectrum.