Well, despite what you may have been told, the truth is that cordless drills don't have as much power as corded drills, and percussion type hammer drills just don't deliver the powerful hammering action that the piston in cylinder arrangement in rotary hammers do. So, in a nutshell, your cordless hammer drill is just not hammering hard enough on the concrete with each stroke to do any damage to it.
Understanding the answer lies in the difference in the way twist drill bits and masonary drill bits work:
Twist drill use a sharp cutting edge at the front of the bit to CUT a hole in wood or metal. A masonary bit works by BATTERING a hole in the masonary. To do that, it has to hit the concrete hard enough to bash the concrete into pieces. Percussion type hammer drills have a very high number of blows per minute, but each blow is only effective if it bashes the masonary in front of the bit into dust. So, percussion type hammer drills are all you need for relatively soft masonary like bricks, blocks or mortar because those materials are soft and batter into dust easily. Once you get into concrete or hard rocks, then you need a much more powerful hammering action to make dust out of that stuff. 50,000 blows per minute will get you nowhere if none of those blows are powerful enough to do any damage to the concrete. Rotary hammers have far fewer blows per minute, but each blow is powerful enough to bash the concrete into pieces, and that's why rotary hammers work so much better in these harder masonary materials.
I have a Makita corded percussion type hammer drill that delivers 49,000 blows per minute and it sounds like a screaming Ninja when I'm using it. It's OK for small holes in relatively soft masonary, but a 5/16 inch hole in concrete would be about the most I would expect of it. It drills into concrete, but quite slowly. That's cuz each blow doesn't do much damage.
I also have a Hilti TE-10 rotary hammer that uses SDS drill bits and delivers 400 blows per minute, and that sucker will drill a 5/16 inch hole in concrete about as fast as a cordless drill can drill a 5/16 inch hole in wood. And, it's all because each blow is powerful enough to bash some concrete into dust.
Rotary Hammer Corner
PS: Before they had electric tools, the way you would drill a hole in hard rock (to set dynamite charges, say) would be with two men. One man would hold a tool with a bit on the front of it that looked like any one of those below:
and the second man would hammer on the top of the tool with a sledge hammer. After each blow, the first man would turn the tool so the teeth on the front of the tool would always hit in a different spot. The tool bit would essentially batter a hole through the rock one blow at a time. That's still the way modern rotary hammers work, but just with two employees less.
(All of the above are bits for "rock hammers", which are the pneumatic drills used in mines. Note that each bit has at least one hole in it. That's because these bits use compressed air inside the hollow drill shafts to blow the dust created out of the hole. The compressed air does the same job as the spiral flutes on a twist drill bit or a masonary drill bit.)