I dunno ...
Women do put their purse down somewhere and walk off momentarily, or else they have so much sh*t stuffed into their purses they can't easily grab a ringing cellphone immediately;
One might put their phone down on the table and walk off for a while, not realizing that you might get an incoming call during the period the phone is "unattended";
You forget to (or deliberately don't) take your cellphone with you when you go to the bathroom;
You can't answer the phone even one within your reach because you are involved with something else at that moment;
You might be on the phone with another call and just can't switch over to answer the call-waiting beep, because the first call is an important call (maybe you are on hold, and you just don't want to leave the first call in case you are taken off hold at the moment you have answered the new beeping call);
and so forth.
There are numerous reasons why one can't (or won't) answer their inging cellphone at the moment it rings (or beeps or vibrates).
If you have to turn your phone off, most cellular carriers will immediately send the incoming call to voicemail, or in the absence of voicemail service on the called line (and there are those who never did get voicemail when they first subscribed to cellular service), the caller will be immediately sent to a "vacant" announcement indicating that the desired party is either not available, or has roamed out of any available coverage/service area.
If you have your phone turned on (and are in a signal/service area), your phone will ring about four times (these days, your phone could even ring six or more times), before the call is sent to voicemail, or in the absence of voicemail, to a "vacant" type announcement described above.
I don't know about all carriers, but some carriers do NOT give you a "caller-ID log" of incoming calls during the time your phone is turned off. If a caller doesn't choose to leave voicemail when my phone is turned off, I have no way of knowing that I had an attempt at an incoming call. So ... I leave my phone turned on at just about all times. I put it in "vibrate" mode if I am in a library, in a theater, at church, or other places where it would be rude for an incoming call to ring. That way, even if I don't answer an incoming "vibrating" call, and the caller doesn't leave voicemail, I can at least know that I had an incoming call from such-and-such a number (if the number is deliverable) at such-and-such a time.
If I can "politely" answer the incoming vibrating call and talk in a very low voice, I will. If I need to "force" the incoming call to voicemail, the caller might hear one or two spurts of "ringing" tone before being sent on to voicemail. If I can't even do that, then the phone will quietly vibrate on my end with the calling party hearing "ringing" tone, for about six "ring" cycles, until the cell switch finally sends the call over to voicemail. The caller can choose to leave voicemail or not, but at least I have seen something about an incoming call, in the incoming Caller-ID Log, or even at the moment the call is ringing (vibrating).
So, I don't consider it one bit "odd" that calls to a cellphone could ring several times unanswered, ultimately going to voicemail, or else going to a "vacant" announcement (called party is not available or has roamed outside of the coverage area). We all depend on our (and others') cellphones, but some of us do have other things we are involved with at the moment the cellphone rings, things that take precedance over a cellphone call, just like it with a a ringing landline phone. That's why there is voicemail, answering machines, and answering services. But some people might not even want these things or services on their cellphone or landlines neither.