Because some developers and project managers like, tolerate or even encourage complexity. I've read this comment on hackernews once about it, and it seems to be true.

There is Wirth's law ("software is getting slower more rapidly than hardware is becoming faster").

Generally, developers don't like performance, for some weird reason. The 97% Knuth quote might have played a major role, unfortunately it doesn't apply to game development at all. It's always a compromise between speed and code size.

For the rest, too many developers are not educated about electronics or physics, and often disregard important topics like memory management and data structures. But computer science always is complex. The generalization of garbage collected languages is also a big problem (see Minecraft)

Another problem is the lack of discipline and OSHA-like regulations in software development. The software industry is too anarchic and liberal-minded (Silicon Valley-style venture capitalism has a lot to do with this). It results in a lot of work being easily thrown away.