in a sentence - ' i have my supper by 6'o clock ' why only the preposition ' by ' is used. why cant we use ' at ' instead of it.?

Both the versions are correct.
When you say, "I have my supper by 6'o clock", it means you have your supper on and before 6'o clock.

However, when you say "I have my supper at 6'o clock", it means you have your supper exactly at 6'o clock.