It's not too expensive
Some years ago a 1GB SD card costed 20€, now a 8Gb SD costs 20€
And how much does a DVD cost?
SD cards are fine for people to use as rewritable media. For game distribution you'd either use optical media (DVD/Blu-ray) or ROM media like the DS (plus maybe some flash for gamesaves). But for the capacities that console games require, optical media is still cheaper.
My Mario Kart Wii friend code: 2707-1866-0957