Short answer: Because the cell system doesn't actually know.
Long answer: There are two reasons why the cell system can't do this, and both come down to the fact that it doesn't know that the cell sector you're in is at capacity.
First off, it's not easy to say exactly when a cell sector is "at capacity". CDMA doesn't work that way. Every simultaneous call uses the entire spectrum, with separate encoding. Each call looks like noise to all the others, and every time you add a call to the sector, the noise floor rises for all the others. This decreases the signal-to-noise ratio for all the other calls.
But the amount of noise that each call adds to the noise floor is not constant. It's a function of the location of the phone and thus how much power it needs to use to get its signal through to the cell, and it's also a function of the actual voice traffic it is carrying, which varies from moment to moment.
The theoretical maximum number of calls a sector can carry is 61, because there are 64 Walsh codes (which is how "channels" are differentiated from each other under CDMA, since each channel uses a different Walsh code) and three of them are reserved. This leaves 61 for traffic channels. In practice, however, no cell ever can support 61 calls simultaneously.
But exactly how many they do get is very much dependent on circumstances, and it changes from second to second. Where are all the phones with existing calls? What's the terrain like? Is it raining? How are all the neighboring cells behaving and how busy are they? Which codec is each phone using and what is its bandwidth? Many factors feed in. (I've heard that about 50 simultaneous calls is pretty much all anyone ever gets, but I don't know how true that is.)
So when the cell system tries to create a new call, it doesn't actually know whether the cell can sustain just one more.
But there's another factor which is even more important: the cell system doesn't actually know which cell sector your phone is in. What it knows is which zone you are in, but a zone usually consists of more than one cell, usually having more than one sector each. Any time your phone moves from one zone to another it has to register, which means that it turns its transmitter on and sends a brief message to the cell system announcing some important information about itself. The cell system knows where it received that message, and thus at that moment knows which sector of which cell your phone is locate within. However, thereafter you can wander at will within the whole zone and the cell system won't have the faintest idea where within it you are.
This is actually a good thing. If your phone had to transmit every time you moved from one sector to another, your standby time would suffer badly, because ordinarily phones can perform idle handoffs quite commonly even if the phone is standing still.
Whenever there is an incoming call for your phone, the cell system broadcasts a page on the paging channel of every sector of every cell in the last zone within which it received a registration from you (if it thinks your phone is still turned on). But within that zone, some sectors might be extremely busy and others might have very little traffic. If your phone is in one of the busy sectors, it might miss the page entirely or it might receive it and try to set up a traffic channel – but fail. On the other hand, if it is within one of the less busy sectors, it probably receives the call normally.
So the cell system doesn't know which cell sector you are in, and even if it did it doesn't know whether that sector is at capacity. Therefore it has to attempt to page your phone before it routes the call to voicemail.