1) Games like Terraria are sold as "finished" products but still are constantly updated.
Incorrect. Terraria was released unfinished because a leaked version was posted, so the team released the game early so that people would actually have the choice of buying it opposed to pirating it. The devs confirmed this on terrariaonline.com a while back, and stated 1.0.6 is what they wanted the game to be originally (the other minor patches like 1.0.6.1 being for balance or serious bugfix issues), and that 1.1 will be the first major content update.
Re-Logic sold Terraria unfinished, and give free updates to get the game to a milestone and will then keep rolling out free content.
Mojang sold Minecraft unfinished, and give free updates to get the game to a milestone and will then keep rolling out free content.
There's more parallels than that, actually. One of Terraria's recent updates brought it a potion system, different damage types, and buffs/debuffs, too (since the original version sold was unfinished). And the devs stated that even though the different damage types/elements have been implemented, nothing reacts to them
yet (for example nothing takes extra magic damage or resists fire damage).
3) No, the game is just boring. There's like nothing to do. Building isn't fun. Combat isn't fun. Mining resources isn't fun. Everything is just a monotonous grind with no payoff. Sure, I can build a fancy house, maybe even a town, but for what purpose? To show it off?
Just because you don't enjoy
making things like this doesn't mean that the people who do are bad people.
Maybe it is a giant sandbox but a sandbox isn't fun unless you have toys in it.
Redstone says hi. It's a sandbox game,
build your own toys!
-http://www.youtube.com/watch?v=YB-LdSbshPg
-http://www.youtube.com/watch?v=hUcK6LaBUS4
-http://www.youtube.com/watch?v=7bpAPWIpBwY
-http://www.youtube.com/watch?v=hj6llEGGERk
-http://www.youtube.com/watch?v=IpH8VCoFjew
You may not enjoy making things like that, but many, MANY people do, and you have no right to insult other people for enjoying the things that you don't.
EDIT: Fixed a typo, added some more clarification.