The main issue is that RFID is not accurate enough. Typically off track times are to the nearest second, whereas track needs to be 0.1second minimum usually achieved by hand timing, and 0.01 by electronic timing using other technology.
The advantage of RFID is the ability to handle lots of runners over a long period of time - compared to most track events being numerically. You could handle say 800m plus races, with up to 20 runners finishing a few seconds apart (but not accurately enough), but think of say 8 sprinters potentially finishing within 1second of each.