Because we’re not to measure ourselves by the standard of the world.
God formed us exactly how He wanted us to be, and He is still shaping us as our faith strengthens in Him. He designed our body-type, our personality, our intellectual capacity, everything about us. As we’re transformed by the renewing of our minds, He’s reshaping us to make us more Christlike. Not on of us is the same, each of us having different skills and talents and quirks.
We were each planned by the Creator of the universe. We’re not an accident or a creation throw together at the last minute. He put thought, precious thought, into us. How He measures us differs from person to person. We don’t need to measure ourselves in comparison to someone else’s success or failure, but to our own God-given potential. It’s like in school, when everyone fails a test, but you scored the highest failing grade. Don’t celebrate because you did the best out of your peers if you didn’t do the best out of what you are capable of. In the same way, don’t consider yourself a better or worse Christian in comparison to someone else’s walk. That’s their walk, the path the God has chosen for them. Don’t worry about how strong or weak their faith is compared to your’s. We need to focus on being who God has called us to be, not on being who He called someone else to be.