they do eventually. but at first they have a large enough VSD to help a left to right shunt. cyanosis develops when the shunt becomes right to left. a left to right shunt means that less oxygenated blood is pumped out the systemic circulation. but with a right to left shunt, the blood has less a chance to get oxygenated n so de-oxygenated blood gets pumped into the circulation. so with tetralogy of fallot where the VSD is large enough to allow a left to right shunt allows the child to get some oxygenated blood.
perhaps someone else could give a more detailed explanation.
Tetrology of Fallot is the most common Cyanotic heart defect. The main defect is the failure of the A-P septum to align properly leading to pulmonary stenosis.
The severity of symptoms depends the degree of pulmonary stenosis. Some children with TOF may have normal appearance at birth called "Pink Tets" but later in life develop cyanosis due to right ventricular hypertrophy.