That's fair, but do you put hundreds of hours into them after one or two playthroughs? The games are relatively short, but the loot table and build system is what keeps a lot of people playing and grinding out hours through dozens of playthroughs just to tweak out all of their characters.
While I don't like the normal-vault hunter-ultimate playthrough design, I don't imagine that most players that still like the game have the patience to go through all of those play modes over and over and over again, and on multiple playthroughs, if they don't like collecting high end loot.
The systems are different, too. I much prefer BL1 loot table to BL2, even though I probably put more time into BL2 (better characters overall, even though I still think Lilith from BL1 is the best of the series). BL1 offers more variety and much less grind for getting desired gear, because BL2 and TPS system pretty much requires you to get Legendary gear if you want to max out your builds--there are a few exceptions with some gear, of course.
at least, that's my assumption.