This simple answer ^ is probably close to accurate, but I'd modify it a bit.  We keep them, not so much because we 
can, but because we 
want to,  i.e. we are motivated by pure selfishness.
The justification for anything, done by anyone, can be boiled down to that one simple constant: selfishness.  Anything we do...we do for selfish reasons.  People who steal do it for entirely obvious selfish reasons: because they want the loot, and having it makes them feel good.  But people who live  pious lives and give to charity and volunteer at soup kitchens and rescue sick fish and chain themselves to old growth trees and perform any number of other "selfless" acts...do all that stuff because it makes them feel good.  They've been raised to believe that good is good and evil is evil, and by  doing good they will earn some Brownie points with God and go to Heaven...in other words, they do it ultimately to benefit themselves; they're being selfish.
Mother Theresa was one of the most selfish  people who ever lived.  She decided, or rather was  raised to believe, that she needed to help everybody because that was God's will and she was doing God's work.  But she didn't do it merely because she thought God wanted her to; she ultimately did it  because she would feel good about herself for getting in good with God.  Selfish.
Born-again evangelists who climb over my gate and  trudge up to my house to try and coerce or implore or threaten me to see things their way...are selfishly grasping for that 
rush  of gratification they have been taught to feel when they save the soul of another heathen or sinner.  When a few deep-throated threatening barks from Duke sends them scurrying back to their cars...well, that selfish self-serving reaction takes precedence over the other selfishness.  People who log onto MFK and answer silly basic questions posed by silly newbie fishkeepers don't do so merely to be helpful; they do so because they have been taught that helping is a good deed, and doing "good" will make them feel better.  All selfish.
IMHO, that makes about as much sense as asking a nun for advice about the merits of various sex positions.
Yep!  We eat meat because we want to.  Same with fishkeeping.  Entirely selfish.
Everyone who wants to pile on and tell me how wrong I am, feel free to proceed.  You'll feel good...you selfish bastidges...