I've seen people who have learned NCERT thoroughly but did not clear the NEET exams with good marks, What's the reason with them?

If you want to score good marks in NEET alongwith NCERT you have to refer other books as well. NCERT is helpful to build your base strong and that your all concepts are clear. You can check below recommended books for NEET preparation- Best Biology Books for AIPMT/NEET Preparation: NCERT Biology of Class XI and XII Trueman's Biology Vol 1 and 2 Objective Biology by Dinesh Pradeep Publication's Biology GRB Bathla's Publication's for Biology Best Physics Books for AIPMT/NEET Preparation: Concepts of Physics by H. C. Verma NCERT- Physics Part 1 and Part 2 D C Pandey Objective Physics Fundamentals of Physics by Halliday, Resnick and Walker Problems in General Physics by I. E. Irodov Best Chemistry Books for AIPMT/NEET Preparation: NCERT Chemistry Physical Chemistry by O.P. Tandon Organic Chemistry by Morrison and Boyd for Organic Chemistry Modern's ABC of Chemistry for class 11th and 12th Concise Inorganic Chemistry by J. D. Lee for Inorganic Chemistry Dinesh Chemistry You should also refer past year problems to understand the exam pattern and level of questions. Try to solve as many as complete mock test series for your preparation and to know your strength and weakness