Routes were traditionally built using SIDs, airways and STARs, not individual waypoints (aka fixes).
From the departure airport, there will typically be multiple Standard Instrument Departure routes, each spreading out to several "transition" fixes. At the other end, there will be similar Standard Terminal Arrival Routes and transition fixes.
Then you find the best set of airways that lead from the chosen SID transition fix to the chosen STAR transition fix. For brevity, you only list specific fixes where you will change from one route to another.
Note that the shortest path in time (which should also require the least fuel) may not be the shortest in distance due to differences in winds aloft, and some potential routes may be eliminated entirely due to passing through severe weather.
The new "Free Flight" concept is touted as allowing every flight to make up its own route (at least between the transitions) out of seemingly random fixes, i.e. not using airways, but that is currently only available in relatively uncongested areas or altitudes rather than the norm.