The reason why the oscillator needs to get noise in order for the oscillations to start is that in real life , you'd build an oscillator and once you built it you'd connect and turn on the DC voltage supply for the operational amplifier , switching the power supply on causes a side effect of noise which starts the oscillations. Since our op-amp in everycircuit always has the voltage supply connected to it and we can just change it's value hence there is never a noise in the system until we insert one ourselves.
After inserting noise you should know that B*A>1 once you turn the oscillator on and you'd see that once you insert the noise the amplification would start enlarging it until it gets to the power supply's limit.
