The bulb has nothing to do with starting. When you turn on the key, the bulb gets power from the switch and ground from the alternator so it lights up. When the alternator kicks in, it gets power from both directions and no ground so it doesn't light.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.