Because, while you can beat the CPU on most difficulties by button mashing, other players learn ways to counter most attacks. Liu Kang in particular, is a really good character against button mashers. Also, Scorpion (who I'm assuming you used) is very much a beginner-friendly character who's good at all ranges. Try playing as a ranged attacker like Ermac and you'll see the difference. Difficulty changes are also much more notable in fighting games, fighters you can absolutely destroy on Easy can kill you on Medium. That's not even getting into the bosses.
And as for my opinion on MKX, it's definitely a slower-paced fighter than 9, I've had little to no trouble chaining combos and specials (especially with Mileena). Personally, I like the new pace, feels like Netherrealm realized that there were people like me who enjoyed Deadly Alliance and Deception and worked to mix those two in with 9 to create X. I especially enjoy that I'm not frantically mashing buttons, hoping for something magic to happen while I'm being combo'd into oblivion as often. The story is also good, especially for a fighting game. They did a great job with new cast, each of the new generation kombatants feel like their own characters, rather than lazy clones of others. I'm also very impressed that they managed to take the underwhelming MK4's story and lackluster big bad and turn both into strong aspects of a great game. Shinnok's still no Shang Tsung or Shao Kahn, but he's a lot more menacing now.
So yeah, I think people should buy it.