Yes modern EBRT can be focused very much on the area of interest. But there are lymph nodes all over your pelvis and any of them may have cancer so the area of interest is the whole pelvis. If they try and focus away from the rectum they will miss lymph nodes near the rectum. So if you want to RT all lymph nodes in the pelvis you need to irradiated the whole pelvis.
It takes about 15 minutes to deliver the targeted RT. If you were absolutely certain you knew where each node was, and there are probably about 100, you would be on the treatment table for 25 hours per day. Quite literally there aren't enough hours in the day for that level of treatment.
Targeting every small lymph node individually would be impossible. They are too small and unless you know the position exactly you would keep missing them and just hit healthy tissue.