Every 14 days, a DHCP lease renewal takes place that acts as a handshake between the ISP and a household modem. If the connection is still valid, the ISP will not disrupt service via providing a new IP address. There are multiple reasons they do not issue a new IP every time this lease renewal takes place:
The removal and re-provisioning of an IP disrupt service to the household.
There are hardware and bandwidth costs to execute these processes. Then there are support costs to deal with the lapses in service when this takes place.