I can see that if a previous user of Uber had failed to pay, had cancelled payment after calling for service, or had used a stolen credit/debit card using that particular iPhone they would be unlikely to want to serve that person calling again from that same iPhone. However, people do sell iPhones to innocent buyers and to use the immutable IMEI assigned to the iPhone would be unfair to the buyer.
Ah, makes sense now, assuming that IMEI/MEID are truly immutable.