I keep saying this criticism about the NRS era and it completely baffles me. The last time the heroes had a clean, unambiguous victory was MK4 all the way back in 1997. Since the 2000s the heroes either lose (Deadly Alliance, Armageddon) or their victory comes at a terrible cost (Deception, 9 through 11). Yeah, they tend to lose in cutscenes but a good villain isn't just dangerous because they are good fighters. Fire God Liu Kang only made the most headway because he started using strategy rather than just relying on martial skill.
You answered your own question. Because they lose ridiculously absurd situations that makes them feel very weak at times.
-Kung Lao beat Shang Tsung and Quan Chi at the same time.
-Stryker beat Kintaro 1 on 1 right after he just beat Mileena 1 on 1 right after he just beat Reptile 1 on 1 back to back to back.
-Speaking of which, Cyber-Sub beat Goro and Kintaro at the same time.
-Etc
All that was just MK9 lol. MKX didn't do any better.
-Kung Jin whooping the new Outworld ruler and making him submit. Jacqui also washed him up later.
-Sonya beating Quan Chi so bad she went full God of War 3
-Sindel killed half the heroes but a washed up retired old Jax beat her 1 on 1. Then he also beat Liu Kang.
-Cassie Cage giving the villain roster the 2006 John Cena treatment.
I could go on, but you get it. There are other ways to do the story if they wanted that wouldn't involve making the villains so weak.