This is a Semi Controversial Topic.

A LOT of people think it doesn't exist and I would like to know why?

It's pretty clearly stated and established throughout the entirety of Bleach.

  
Reiatsu Negation is when a Spiritual Attack is negated by the Person being attacked using their own Spiritual Energy. Aizen didn't actually first do this, he simply explained it in a more elaborate way compared to KENPACHI who ACTUALLY explained it FIRST.

  
Aizen negating Soi Fons Shikai is the first time we see it being used via HAX. We only see it being used with Pure Energy or Physical attacks during the series. Arguably Yoruichi used it against Soi Fon during their fight where she completely stops Soi Fon in her tracks.

  
Another example is when Ulquiorras Sheer Reiatsu in his Resureccion was able to completely negate Ichigos Full Powered Hollowfication + Bankai Black Getsuga Tenshou with Zero Difficulty. 

  
A further example is when Ichigo is unaffected by Kurohitsugi's Gravitational Pull during his fight against Aizens Hogyoku Fused form.

  
There are just too many examples to say Reiatsu Negation does not exist. However, I CAN see the skepticism when it comes to Hax based abilities as Aizen may have been using Kyoka Suigetsu against Soi Fon but itself very clear that he left the battle around that moment. They obviously weren't all fighting Hinamori as she'd be dead, Hinamori was most likely put there at the very last second. 

  
Also, let's not forget that Aizen is capable of using Kaido like most Shinigami as it's made clear that Aizen Mastered the Arts therefore he most likely had some Kaido knowledge. Plus had the Hogyoku, so any minor injury such as Shinjis slice or Soi Fon stabbing him could be easily healed. Also the Hogyoku probably just Healed him anyway during this time as Yamamoto's Shikai had seared him, then afterwards Aizen is shown to be fine. 

  


A Topic We Need to Discuss: Reiatsu Negation