But by your logic no professional footballer has any margin for error. A strikers job is to score so would you expect every striker to have a 100% conversation rate otherwise they are no good?
No, that's not the logic at all.
Firstly, it's two different positions. A 'keeper is much less involved in general play than an outfield player, therefore a mistake is correspondingly a lot more meaningful.
Leading on for that, conversion rate IS a key stat for a striker, but it doesn't have to be 100% obviously - even 50% for a striker is very, very good.
But if a 'keeper only catches a ball 50% of the time it's very, very bad. That's the difference.