I'm with the crowd that explains it as a desire to drink decent beer without paying full price- both fine engineering traits!
After a couple batches, you realize that it takes beer to make beer, at which point it becomes pretty much impossible to stop...
Me, I had to stop my kit-brewing right after grad school. Once I no longer had sufficient buddies to brew and drink with, I found I couldn't drink it fast enough to keep it "fresh" without becoming considerably more of a man than I am now- weight-wise I mean. Just one belt of "skunky" homebrew left too long on the shelf has you racing for a razor to shave your tongue to get that horrible taste out of your mouth...
Fortunately for both of us, my DW doesn't drink beer so getting her "help" wasn't an option. And inviting friends over to drink it all (without also helping to pay for the kits, brew and bottle it) cut too much into the "avoiding paying full price" goal. So that put an end to my zymurgical aspirations!