They have special deals with the telcos that TCP connections to their servers can have longer TTLs. Plus of course, even if you do polling, if you have 5 different apps that connect every 10 minutes to their own respective services with different offsets, you have the device waking up every 2 minutes, with a potential latency of up to 10 minutes for each individual service. If it uses a central push service, you can set it to 5 minutes, the device would therefore wake up less than half of the time, and maximum latency would be 5 minutes, not 10.

Thus, it makes total sense to have one mechanism for push messages for the entire device. It's great to have degoogled alternatives here.