There are a few reasons as to why you may not be in my contacts, and I will address those reasons here. If there are other reasons, I will add them in this page. The three main reasons are not knowing your full name, lack of any real contacts, or severe ties on bad terms. I will go into more detail in the following paragraphs.
The first main reason will be that I do not know your full name. In example, I know 2 Joes, so trying to figure out which Joe will I call if I don’t have surnames will be at best challenging. And let’s face it, you have my surname, so it would just make sense if I have yours. If you want to be in my contacts, you would want to send your full name, and all relevant contact information.
The second reason is that it might have been a very long time since we had any contact. With exception of family, and medical professionals, if I have no contact with you for 6 months, then I am likely going to consider taking you out of my contacts. This is to reduce the number of ghost contacts I have, and make it easier to find the contact that I am in touch with. And think of it, if it has been more than 6 months since we had any contact, do we really have any sort of relationship?
The third reason is there has been a bad severe of relationships between us. If we are not going to be friendly with one another, than there is no point in keeping you in my contacts. This is a rarity of why I would not have you in my contacts, but a perceivable possibility.