There are several reasons for the length of the tracks in Pure and Pulse (which are similar to Pure in length)
Some reasons are technical, longer tracks simply take up more memory due to increase in texture size / quantity and polygons etc, to retain the high quality look you have to set an upper limit and work to it. X amount of polygons in the track and environment, spread over 5km of track will look better than the same amount spread of 10km
Another reason is preference, the designers who make the tracks have preferences. Personally I like short tracks because they are easier to memorise and offer really cool opportunity for shaving corners and identifying exact spots to make up time in time trials.
I also think they can be extremely intense experiences.
My own personal belief is that longer tracks ultimately do not lend themselves as well to time trial events as shorter tracks. I am sure some people will disagree, but thats a preference. Though I think Longer tracks can make for ace race experiences.
Having said that one of the other designers has a thing for more technical tracks etc which can include longer tracks. We do try to have a balance and a range, we dont set all the tracks at 4km. Indeed in Pure including all the downloads the average length was over 5km as I recall (I think Sinucit was the longest at close to 6.5km)
Ultimately though we dont judge a track on its length or on how long it takes to do a lap, its judged on how it plays and flows and how it tests the player. we may well push the track length a little longer in some cases in future especially as technology improves it becomes a viable option, but nothing is done for the sake of it.