Why do they take xrays from the teeth when the patients are in the reclining position ?

Not always: This is not always the practice. In the us, most dentist take the radiographs in an upright seated position. It probably just depends on what your dentist and their staff are most comfortable doing.

They don't have to : Be taken with the patient in a lying down position. We typically takes ours with patients sitting up, but if an intra-operative xray is needed, we might take it lying down. What is most important is the correct angulation and accuracy of the image and not the position the patient was in when the image was taken.