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