It's weird. I only played GTA IV multiplayer a couple of times because it was filled with mindless trigger-happy fools, as expected, and it was pointless. But I was able to keep playing / replaying GTA IVsingle-player as Niko and always found things to do, and to make new challenges for myself, and it seemed like I would just explore and still find new things. Then when TBOGT hit, that was my favorite for the rest of the years until RDR came out.
But the design of GTAV just does not lend itself to replay. The few mini-games which require you to scour the map for scraps of paper, real estate signs or spaceship parts are just uninspired and tedious. And even though the map is big and beautifully detailed, it is rare that I see a photo in Snapmatic that is a place that I don't already recognize - it's strange that we are able to get so familiar with the map so quickly. So, strangely, even though I hate multiplayer, it has attracted me there to GTA0, just so I can keep replaying the game. The user-content races have further upped-the GTA0 interest level.
One of the best (and unintended) exploration activities in GTAV, is following the GTAV mapping thread in this forum. That was fascinating before the game release, and is still fascinating now.
Bottom line is that Aaron Garbut and his art team did an unprecedented virtual world design. But in those same 5 years while the GTAV world was being built and detailed, it seems like the game designers and story designers just phoned in their work in the last 12 months and nobody was enforcing the game design quality.