Because God, in His inscrutable wisdom, has chosen to put treasure in clay pots.
I can't be a superstar, but I can be a pot.
Remember the story of Gideon, and how he and his rag-tag army took on the Midianites? Their lights were hidden in clay pots until the right moment, and then they smashed the pots to let the light shine. God then miraculously routed the enemy before them.
God has given me plenty of good hard whacks in my life, and at times I've felt like he's completely smashed me for good. Maybe one of His purposes for doing that is to let some light out.
I don't have any light of my own, but He is the light, and it's my hope that He'll shine through just a bit on these pages, "that the excellence of the power may be of God, and not of us."