Why there is no greater natural number?
Sure there are greater numbers. For example, 4 is greater than
3.
If you mean "... no greatest number", the reason is that you can
always add one more, and get a number that is even greater. Thus,
for example, 10 is not the greatest number, because you can add one
and get 11. 11 is not the greatest number either, because you can
add one and get 12. Etc.