Because he's not the butler; he's the bodyguard.
He's both. Remember the line in the hostage scene? When Alfred started to take charge, Simon Stagg looked at him and said, "Who are you?" and he replied, "I'm the butler."
Again, it's much the same as in Johns and Frank's Earth One
graphic novel. There, Alfred is a former British intelligence agent that Thomas brings in as a bodyguard for young Bruce, a role he's reluctant to adopt at first, and when Thomas and Martha are killed and he learns that he was appointed Bruce's legal guardian, he thinks it over for a bit and then goes into the grieving boy and introduces himself as "your butler."
The point is that if you didn't know he was Alfred, you wouldn't know he was Alfred. If he just showed up, without spoilers, you'd be wondering who the big bruiser is.
Which is why I'm smart enough to wait for more information so that I can find out.
There's nothing wrong with being made to wonder. It inspires curiosity, and curiosity is good.
That's part of what makes it pandering. They don't want Alfred because he's an older man who does chores around the house-- he must be re-imagined as a younger man of action!
Obviously you're forgetting many of Alfred's awesome action moments from comics and television. My favorite is in the '66 series' "Flop Goes the Joker." The climax of the episode has the Joker breaking into Wayne Manor to avenge himself on Bruce for some perceived slight. Alfred confronts the Joker, gets into a swordfight with him, and wins
, sending the Joker retreating into the study, where he stumbles upon the Batpoles (which are mercifully unlabeled because Alfred's just repainted them), takes them as an escape tunnel, and slides down. Whereupon Alfred uses the emergency lift controls to keep the Joker from reaching the bottom, and sends the Joker sliding up and down the poles repeatedly. By the time Batman and Robin arrive, the Joker is begging for mercy. Alfred defeated the worst arch-criminal on Earth singlehandedly in five minutes, without the Dynamic Duo needing to do a thing.
In short, if you want Alfred to be some doddering old weakling useless for anything but housework, then you don't know a thing about Alfred. There's more verbiage devoted to him in TV Tropes's Battle Butler
entry than to any other single character (even Hayate the Combat Butler!).
And as I've already pointed out twice, the show is set in the early years of Batman's career. Everyone is therefore younger.