It's simple, batman can beat everybody in every universe (he can fuck up the hulk, Goku, the American army, god) 'cos he has the strongest thing in comics, plot insulation.
The fans love him so he can't lose basically. They say "he is so relatable 'cos he has no super powers but still beats super powered people" which has so many things wrong with it....
Firstly, no super powers? He has apparently mastered many forms of hand to hand combat, which take mere mortals life times to learn... So he has at least a super human ability to learn. He is the most alpha of males if the facts about batman are to be believed. I'm not totally knowledgeable about batmans enemy's but aren't most of them just normal guys with quirks? He can apparently beat superman up but he gets matched against joker, riddler, Mr freeze, poison ivy, scarecrow, Bane and penguin.... Wait, Batman can beat superman but gets his ass kicked by bane?
Second, if batman ever did lose shits would be flipped, tables would be flipped and the Internet would be more pissed than when they learned who the new batman would be. "he lost to Bane", only so he could have a redemption story.
Thirdly, the batman vs superman thing. Superman, known for being faster than a speeding bullet, bullets bounce off his pupils, can hear the most silent of farts from a continent away, can almost life the mjolnir (did I spell that correctly?) and can see through walls... What the fuck would stop superman from making Bruce explode with a punch before batman could even see superman? Nothing but nope "superman is boring so batman wins".
Fourth, how can any of you relate to a fucking multi billionaire? Come on now.