I hear this a lot. Was I the only kid who would build what ever was on the box first, play with that for a day - break it and then have it all go into a big box?
And all the funny bits people moan about make excellent space lasers.
As a kid, I always found the big box of bricks boring. The sets had all the cool pieces (and still do) and, like you, I'd build what's on the box initially but not long after those pieces just went into the pile to build other cool things.
As a kid I used to have tons of Lego. My dad would always bring a set or two back after being abroad for a few weeks, so I had boxes full of Lego. And they were all specific(jigsaw-like) sets - bulldozers, cars, castles, Indian forts, spaceships, that kind of thing. And you know what? Just because the instructions say you have to build a spaceship,doesn't mean you can't disassemble it afterwards. I remember that I would build something,and next day it was back in pieces, contributing to my huge Lego city or whatever it was that I was working on at the moment. For me, sets were just a fun thing to assemble for a day,and then a source of parts after day one.
My later custom designs benefited from having read instructions and putting together the set as intended several times. They gave me ideas about how pieces _could_ fit together.
I don't mean this as a slight, but to a kid there's absolutely nothing lacking from the modern kits. I'd say that the "half-hearted" part is solely in your perspective now as an adult.
"Lego sets are so uncreative nowadays, I buy a set and then only build what was intended. I'm obviously not the problem." Seriously, those sets and the cool pieces just add many more dimensions to what you can creatively build.
Lego Classic has just arrived in the UK but it seems a half-hearted piece of mummery to me.