why are there only seven sins in christianity , while everything is a sin in islam ?

Such is not the case.

All sin is death dealing, read Romans 5:12 then Romans 6:7 and 23.The teaching of seven deadly sin is a false teaching of Christendoms denominations like most of their teachings, celebrations, doctrines and traditions. True Christians reject those falsehoods. 

There is far more than 7 sins in Christianity, so the premise is flawed.

Sin is any willful disobedience to the law of GOD.

False and false.

There's some 613 commandments in the bible and violating any of them is considered a sin in the most literal definition. I think you're thinking of the 7 deadly sins which aren't really in the bible.

Christians want to live  reckless.

Islam what to live pure and righteous.

Bamdad. I might have known.