Why was Karna weak despite being the son of the Sun God?

Why was Karna weak despite being the son of the Sun God? Karna: The behemoth warrior of the sun. He was a excellent warrior. (Maharathi) He was one of the top elite warriors in Kaurava side. He achieved the feat of defeating the 4 Pandavas. Defeated Jarasandha. He caught the Rudra Asani of Ghatorkacha and hurled it back. Despite being defeated by Satyaki and Bhima, Karna managed to defeat them back with more force. Karna overpowered and defeated Pandava commander, Dhrishtadyumna. Karna gave his 100% in Kurukshetra war. Arjuna himself praised Karna’s Bhargava weapon. Karna was a great disciple of Lord Parashurama. He gained all celestial weapons that Lord Parashurama possessed. He was also trained by Guru Kripacharya and Guru Dronacharya. He also has resisted many dangerous group attacks. He was a great commander in chief. He was superior to Kritavarman and Kripacharya. Equally powerful compared to Aswathamma. Had divine bow of Devaraj Indra, Vijaya. He defeated Ghatotkacha and also slayed him using Vasavi shakti. It is unfair to claim that karna was a weak warrior. I never admired Karna but I can't deny that he was a Mighty warrior.