Could you explain why do some dentist say that third molar tooth is weaker than second molar tooth?

Shorter roots ?: That may be one reason. Another is that the further you go back in your mouth, the more force you can exert on the teeth due to the lever action of your jaw.

Third Molars: Usually are smaller and shorter than second and first molars and their roots are usually closer together and more conical in shape. Since the second molar roots are usually more spread apart, they offer more support. It's like spreading your feet apart to brace yourself rather than standing with your two feet together.

Atavistic: Smaller crown and root structure so not as stable as larger 2nd molar. Harder to care for because of their position way back in the mouth. Often poorly angulated and don't function as well. 3rd molars were designed to compensate for tooth material lost because of our prehistoric ancestors unrefined diet. We don't wear our teen out the same way, so 3rd molars not as necessary today.