OP this thread should have been locked for daring to ask such a ridiculous question. But I'll indulge...
You ask why he's a legend other than the fact that WWE tells us he's one. Don't you think that by 25 years the fans would catch on to the fact that we're being fed something that isn't actually true (he's strong, dominant, intimidating, one of the greatest) if it WEREN'T true?
Character:
He's a legend because he took a what should have been a short-span comedy gimmick and made it riveting. He is one of the most underrated performers outside of actually wrestling ever. His voice, his tone, eyes, facial expressions, body language... everything right down to the way he clenches his fists and stares through his opponents before entering the ring are so masterful in performance that fans believe every single thing we're told about him. Then he did a 360 and portrayed a close to life biker gimmick and made it just as successful, and then he transitioned back without missing a step, and the fans loved him just the same.
Cool Factor: He was one of... if not the first (even before Stone Cold) to make a badass heel actually cool. He talked about ripping out viscera and bleeding his opponents dry in cartoon era of little children and for some strange reason he got CHEERED. They loved him without the WWE even trying to push him as something great to be loved or appreciated. He was always just cool. No matter what he was playing heel or face. Even in defeat. His entrances, body language and expressions are pure swag along with the way he carries himself.
Athletic ability: This should speak for itself. The things he can do in the ring for a man of his size are incredible. And despite the nagging injuries and pressure the travel and everything else put on his huge frame he continued to get better through the years.
Ring psychology: Very few can tell a story as good as he can. He has the ability with a very select group to garner certain reactions from the crowd without speaking- a mere tilt of his head, a motion, a facial expression is sometimes enough to convey exactly what he needs to.
His creativity and vision: Vince created the character, Mark Calaway took it to the heights it went to. Everything from his old school darkside promos were scripted largely by him (confirmed by the late Percy "Paul Bearer" Pringle), his outfits, looks, mannerism, habits were all a unique creation from his own mind and vision.
Lived the gimmick: Was he actually someone who buried people? No. But he sure as hell lived the wrestler/rockstar lifestyle of the old partying days. He's as much of a legend for his partying and running around smoking weed, drinking JD by the gallon, bagging the strippers, hanging with pornstars and rockstars, getting into bar fights in his biker days and raising hell with the likes of Godfather and the samoans back in the day ... as he is a legend on screen. Jake the Snake ALWAYS has an Undertaker story to tell in every interview... even he seems to revere Calaway from those days.
The MAN backstage: There is nobody who unequivocally stood for the "boys" backstage more than he did. He is the agreed upon and very enthusiastically-supported backstage leader, the "Godfather" among the wrestlers if you will and nobody disputes this. He was judge of the infamous wrestlers court and yet with all of these he's not considered amongst almost anybody to have been an asshole. Instead the opposite is overwhelmingly told to be the case. Even those who have left the company in bad circumstances and are currently bitter have nothing but great things to say and stories to tell about him. Mark Calaway is spoken about with reverence in shoot interviews and seems to be thought of as much of a legend backstage as the Undertaker is on screen.
Toughness: he's wrestled with broken bones, cuts, rib fractures, torn muscles, massive blood loss, a crushed face, concussions (most recently one so severe it had him barely lucid in the ring and collapse unconscious backstage after) and been blown up in his own pyro fire.. and ALWAYS continued the match and took it like a man, never complaining or expecting accolades or fan-fair.
Longevity and Endurance both on/off-screen- in this brutal, fairweather business where the fans can chew you up and spit you out faster than you can say "boo" he has lasted 30 years (25 in the wwe) with the quality of performances he has, with the level of unshakeable respect he commands from almost everybody, has never abused his position of respect and status and hasn't been fired by Vince. Rather he has amazingly remained one of the most liked and respected... dare I say loved people on a personal level by the famously crusty and hard to get along with billionaire Vince McMahon. This is almost unprecedented.
On a personal level, the business does a horrible number on wrestlers physically and emotionally. Many have a hard time leaving it even in their minds and when they do they suffer drink and drug addictions, personal crises, financial ruin, and often early death. While Calaway's physical struggles have been well-documented he has amazingly remained a humble and down-to-earth guy. He no longer lives the party lifestyle and has left his biker days long behind him. He's by all accounts a happy and domesticated husband and father who is healthy emotionally and has been very good with his money by investing wisely and creating successful business ventures outside of wrestling. He's happier these helping coach his daughter's soccer team than he is partying or being in front of the camera. Again this is often the exception unfortunately for people transitioning out of the business rather than the rule. Especially for those who have lived it as long as he has.
Hopefully you don't STILL have to ask why he's a legend. More than anyone else is the man.... Taker is THE MAN.