The surprising thing about Spelunky and its generation is that you would expect that manually designed templates, even if put together in a sort of random way, would get repetitive or old pretty fast. And this would be the case if Spelunky didn't have what the author called probabilistic tiles and obstacle blocks (see Rogue Legacy for an example where it does get repetitive pretty fast). But even after >100 hours playing it it's still not easy to recognize all templates and to know what to expect next, which keeps the game fresh and new every time you play it.
EDIT: Ok at first there were no moving creatures in it. I see why it is redrawn, now my question would be more like: why is the whole image constantly redrawn? (Dirty rectangle goes a long way).
So: the reason it's constantly redrawn is because GameMaker redraws the whole screen.
But also, it's important to note that GameMaker is meant to be general purpose game engine. Dirty rectangles work great if you're building very specific kinds of games where only small portions of the screen are updating at any time. For a game like Spelunky, which is a platformer where the whole screen moves (aka there's fast "camera" motion), it doesn't provide very large performance boosts at all. I don't work on GM so I can only guess but I'm pretty sure it's a combination of a super-legacy engine (GM is 15 years old and hasn't had a complete rewrite ever) as well as a need to be as flexible as possible.
Ok. It's because from here nothing moves. Using Chrome 30 on Linux Mint.
I wrote a shareware Lode Runner clone on the Amiga 20+ years ago. Mine was definitely a clone, no bones about it. Spelunky is barely related.
