Discs are also quite sensitive to being exposed to sunlight. Burned discs are 10 times as bad in that respect.
Also, even if you try to exert maximum care while handling them, simply handling them will get you some tiny scratches, which of course accumulate over time... CDs are actually not too sensitive to scratches, as long as they're that tiny, but the higher density types (DVD and above) don't take them well at all...
And about this:
Not completely true. I was in a discussion on a Sega forum where Genesis carts with loading times were named. There were many, including NBA jam TE, Vectorman, Air-Buster, Ultimate Mortal Kombat 3, Golden Axe, Sonic 2, Thunder Force II, Red Zone, Michael Jackson's Moonwalker, etc.
Usually loading times with 16-bit game carts were camouflaged with either blank screens or "splash" screens showing a still image. The programmers did not want you to think you were waiting on the game to load, so the screens did not blatantly say "Loading" like newer gen systems.
Usually cartridges don't have load times, but sometimes they have them, like you pointed out. How can that be? It's when they have compressed the data in some way, so it needs to be uncompressed (in the system RAM) in order to be used by the game. I guess that was usually used to save costs in producing the cartridge (being able to use a smaller sized ROM)