The assumption is not really correct. There is is randomization of identity in mobile telephony networks, at least GSM. There is the TMSI (Temporary Mobile Subscriber Identity) that is used to avoid exposing the IMSI over their airwaves. And it is a specification that dates from the early design of GSM.
The problem is that the IMSI still has to be sent in a number of circumstances (switching on the mobile phone, cell handover). Then, it can still be observed but is protected most of the time.
As for the IMEI I don't think you can easily observe it unless you are the network or running a rogue base station to trick devices into connecting to your own network.
So tracking mobile phones is not that easy. Especially if all you can do is passively monitor the airwaves in the vicinity with software defined radio equipment for example.
If you are a government or the network operator, sure you can do more, track people, listen to their calls. Police too (they use Stingrays, which can act as a rogue base station or passive sniffer).
If you are not the network or the government but an amateur, you could still set up a rogue station or spoof an existing one, but that is not very discrete and technically challenging. Think for example about the inability to route incoming calls to your victims.
Keep in mind that the standards are old, upgrading the equipment and the software would be damn expensive. Operators want to recoup their investments, not invest on privacy which is not a profit center. While the current standards can be improved, nobody really has an incentive to tighten things, possibly make the job of law enforcement more difficult.