Why don't tanker aircraft have ejection seats?
When I was flying the KC-135 tankers - A model and Q models - all crew members had parachutes, and all parachute packs had supplemental oxygen bottles that were good for at least 10 minutes - which was enough time to descend to the point where you did not need supplemental oxygen. If you had to bail out, it would be through the crew entry hatch, on the port side just behind the pilot's seat and in front of the boom operator's seat. If you had to pull the handle for bailout, the crew entry hatch would be jettisoned and a metal air dam would be pneumatically deployed about 4 feet down at the leading edge of the hatchway, to provide a wind block to allow crew members to fall free of the aircraft before getting into the slipstream. This prevented crew members from being blown back onto the body of the aircraft. 

I have never heard of any tanker crew ever having to bail out of a KC-135, so perhaps this is why they got rid of the parachutes? Of course, I retired more than 20 years ago, and much has changed since then. But the KC-135s are still flying, even though they are older than anyone flying them today. 

To answer the original question - both the KC-135 and the KC-46 are modifications of civilian aircraft, which do not have ejection seats; whereas fighters and bombers are designed for their specific mission. 

Finally, I'm not aware of any cargo aircraft that have ejection seats, either.