I was thinking earlier on about AD&D 2nd edition, which is the iteration of D&D I probably played the most during my formative years. It occurred to me that for a long time the only books I had were the DMG and the Monstrous Manual. I was only about 12, and didn't have a part time job, and my pocket money was at most about £5 a week, and the money I did have was being mostly spent on Warhammer plastic skeletons, so I couldn't afford the PHB. I just ran games using the classes from Red Box Basic and tried to guess at what the abilities of rangers and paladins were; likewise, we just cribbed the combat rules from the Red Box.
So from the very start, I've been used to house ruling and coming up with decisions on the fly. There was no question of playing the rules as written because we didn't know what they were.
Without trying to sound too much like a grumpy old fart from Yorkshire, kids nowadays probably don't experience that kind of making-do mentality that we had to put up with, because kids nowadays tend to want for much less than people of my generation did (which is of course more true the further back you go).
I wonder if, then, there is a partial economic explanation for the fact that "DIY D&D" (rather than complete and all encompassing rule systems like 3rd and 4th edition) is so popular with those who are somewhat older, above and beyond the obvious notion that it is based on nostalgia and familiarity.