OK take Borderlands for instance and you see from Steam Global stats of all the people that purchased the game only 67.5% achieved the “Paid in Fyrestone - Completed 5 missions in the Arid Badlands” achievement. So basically 32.5% did not play it (maybe purchased it twice as part of a bundle… who knows). Of all the people who purchased the game only 22% achieved the “Destroyed the Destroyer” achievement so if we adjust that to represent the people who actually started the game to those who completed at least one playthough with one character we have 22% / .675 = 32.5%. So less than 1/3 of the people who started the game make it though the first playthough with one character once. It is a very small minority who get obsessed with the game to keep playing it over and over.
Similarly for TPS we have 85% who achieved the “Invaders must die” achievement only 40.% achieved the “Vault Hunter Superior” achievement awarded in the last story mission so that is aroung 40%/.85 = 47% of those who started the game completed one play though at least once. This quickly drops of though as not many people are going much farther as only 13% achieve the “Space Lord” achievement and that is getting to level 50 at least once with one character. The onto claptastic voyage we see that only 9% who played achieved the “Shadow of Your Former Self” badge that you get when you defeat 5H4D0W-TP in claptastic voyage.
There are similar stats you can work out for BL2 also but basically only the most loyal fans returned for TPS and from those most played it hardly at all. This can also be seen from the quick drop off of player numbers on steam charts.
The significant majority who played the Borderlands games did not complete one playthough on one character even once.

