While there is no definite explanation as to why The Rock lost to Brock Lesnar at WWE SummerSlam 2002, it is to be believed that the decision was taken by WWE as The Rock was already on his way out of the company.
By the time The Rock had his match against Lesnar at SummerSlam, fans already knew he was leaving WWE to pursue an acting career in Hollywood.
At SummerSlam 2002, The Rock put his WWE Undisputed Championship on the line against Brock Lesnar. The Great One lost the title after interference from Lesnar's manager Paul Heyman. With this defeat, The Rock also lost the title of being the youngest WWE Champion ever.
During his showdown with The Beast Incarnate, The Rock received a negative response from the WWE Universe. During his post-match speech, the crowd still booed him. The Rock eventually left WWE and took time off to start his acting career.
In a similar fashion, Brock Lesnar also took time off after a few years. The Beast Incarnate transitioned into the Mixed Martial Arts world and signed with the UFC. Brock made his return to WWE in 2012 and has been around ever since.
Brock Lesnar is currently with WWE and recently failed to win the WWE Universal Championship
At the recent WWE Crown Jewel 2021 event, Brock Lesnar failed to win the Universal Championship from Roman Reigns. After a hard-fought match, Reigns' fellow Bloodline stablemates, Jimmy and Jey Uso, came to The Tribal Chief's aid.
The Usos took out Brock Lesnar with a double superkick and created an opening for Roman Reigns to secure the win and retain the Universal Championship.