Because of pride; ego; self-confidence instead of having confidence in God that He will supply all of our needs "according to His riches in glory by Christ Jesus. We can never do for ourselves what God can do for us, it is futile to think that we can. After all our Father is the owner of the cattle on a thousand hills (Ps. 50:10), we can't beat God giving no matter how hard we try. We should ask God to supply our daily needs because He knows what we need for the day. We may not have anything to eat but a bologna sandwich but we should thank God for it just as if it were a steak dinner with dessert. When He sees that He can trust us to thank Him for a little, He knows that He can trust us to thank Him for a lot. I'm learning that in studying the Lord's Prayer that I'm searching the Scriptures for spiritual revelation and I believe that is why Jesus gave it to us, for us to dig deep into it, to get the meat out of it. In other words it's short and sweet but it's designed for us to seek and mature in the Word of God. This is awesome, I love it!!