Why aren't there brake lights on aircraft?
They simply aren't necessary.

Brake lights are on road vehicles because often they travel at relatively high speeds, and follow relatively close to one another. If a driver suddenly slows, the brake lights help provide a visual cue (which grabs the attention of other drivers, and is visible from quite a long distance) that the vehicle intends to slow or stop.

In aviation, no aircraft should be traveling on the ground (taxiing) at such a velocity that stopping in a short distance is a problem - nor should they be following another aircraft too closely (regulations/airport rules govern both taxi speed and following distances). Taxiing aircraft are either in communication with an air traffic controller, or each other, making the need for sudden stops, change in speed or direction highly unusual. In addition, there is a higher degree of professionalism within the aviation community than your ordinary automobile driver; pilots pay closer attention to what their and other aircraft are doing; i.e., you won't find a pilot with a Big-Gulp in one hand, a shaving razor in the other, head facing the back seat passengers, all while attempting to pilot the aircraft!

In short, brake lights wouldn't be necessary on road vehicles either under the same conditions - but as it is, they assist in preventing collisions in numerous situations. In aviation, these sorts of collisions do not occur often due to the factors above, and therefore are not necessary.