For the same reason a Michelin chef wouldn’t enjoy being a fry cook at McDonalds. It’s all the tedious parts of the job without the challenge and creativity parts.

Are the top movie scores really that bad/simple compared to the average classical piece? Clearly, the classical masterpieces are a step or two above, but they don't always play that or it must get boring quickly I assume.

To me, if felt more like a Michelin chef having a go at a burger. You wouldn't do that all the time, but it could be a fun distraction once in a while. If anything, it feels like you would need quite a bit of creativity to adapt a piece that was not meant for a classical orchestra.