My impression is that it is a licensing nightmare to take on a comprehensive hits collection. When Rhino did the Just Can't Get Enough series for example, they had the rights to those songs for only X number of years -- just long enough to get them out. In fact by the time the last volumes were being released, the licensing rights for the songs on the earlier volumes had already expired, so they couldn't make more of the earlier volumes -- they couldn't ever make a "complete box set". Most stores probably never had all 15 volumes at the same time. There was just a brief window for each volume.
What you tend to find are many 1 or 2 disc hits collections that present the same old songs -- songs which the artists license very often, and very casually.