Who says they aren't using compression? Chances are (as with most games), level data is packed into an enormous file and at least run-length encoded, possibly more. It is probably faster to decompress data on the fly than to load the full data off the disk. Perhaps you mean lossy compression. But again, who says they're not doing it already?
As for laziness, I don't think so. Kojima would snap his fingers and his programmers would do what they're told.
Perhaps it just didn't occur to anyone that mandatory installs are an increasingly hated facet of PS3 gaming. My own feeling is that if they can have a detailed render of Snake smoking a cigarette while the preinstall is happening, they could have cached data within the game. There is plenty of down time in the game - logos during startup, menus, cutscenes, lulls in the action where they could have spawned a background thread to be caching data. Perhaps it might have still have meant occasional progress bars if the caching was not completed, but I still think that is far preferable to forcing people to sit there for 10 minutes.
