Resistors in a transistor circuit
why are R1 and R2 in parallel?

R1 and R2 are not in parallel. However, the impedance provided by R1 and R2 to the input is found by the parallel resistance formula. This is because the power supply has a much lower impedance than either R1 or R2. So, in calculating the input impedance, we treat the (positive) power rail as if it were ground. Then, in our imaginary world where the (positive) power rail is ground, R1 and R2 do become parallel.

shouldn’t there also be a large impedance due to the gain of the transistor?

Yes, there is. But the author(s) chose to ignore that detail in this particular case. The impedance provided by the path through the base would also be in parallel with the impedances of R1 and R2. In this case, it is probably just as significant as the impedance of R1. However, R2 is more significant that either R1 or the impedance through the base.