In response to the OP
For one major reason, and one major reason alone: Because people want to sound cool by hating on the popular thing.
There's minor reasons, though, and one of them are that it's because people tend to think it's nothing more than:
Sky Captanio said:
"Day six in the house. Some of the housemates are in the kitchen. Still nothing has happened."
a view held exclusively by people who have only watched enough of it to fill... well, the time it takes to say the quoted part of that post out loud.
The broader storyline in a Big Brother, Survivor or any reality show that involves a lot of people in not a lot of space doesn't really follow the same construction that a more scripted story does, and as such can't be viewed in the same way, and because of that it gets seen as 'nothing happening', and as a result simplistic, and (ugh) "dumbed down", despite being much more complex because things that do happen aren't so obviously shown to the viewer in the way it might in a drama series.
For example: If you take a show with the same number of main characters as a reality show might, such as a British soap opera, you'll find that most of them don't really interact with one another- Families A and B might be having a massive feud, and the boy from Family B might be dating the girl from Family C, but nobody in Family C will be dealing with Family A- Family C have their own storyline to deal with involving Family D, who won't deal with A or B at all.
The same situation in a reality show would involve everyone- if Housemates A and B don't get on, B and C are screwing and C and D have some task set by the producers, if it blows up between A and B at some point, there's every chance C will get dragged into it and D winds up having to handle the fallout when they're trying to do the task.
In that paragraph it's a mess, now imagine trying to edit it into a TV show when you don't have control over timing, and you can't be sure that Housemates E through to L aren't going to do something else at the same time- as opposed to in a script where E-L basically don't exist for the purposes of that event- it's going to be a difficult story to convey.
On top of this, it means viewers can't just 'dip in'- the A/B plot in a soap can be condensed into two weeks' worth of episodes, finished with and discarded completely, while in a reality show it might play out over the course of the whole series, while other plotlines go on around it- so it looks like nothing's happening, when there's just as much happening, just played out slower and amongst other things- you know why something's significant in a scripted show, because it's relevant to the thing you were just shown 30 seconds ago. In a reality show, you might not realise it's significant, because it's relevant to something that happened three episodes ago that you missed- or worse, wasn't shown anyway because the producers didn't think it'd become anything.
It's an image problem, rather than anything inherently wrong with the form.
The good thing is, though, that this did allow drama writers to take more risks with their scripts- most TV shows used to be very simple in terms of script, even more so than the soap example above, and aren't comparable to the drama we see on TV today.
In fact, any of the more long-form drama that's seen all the critical acclaim in the last decade or so- Lost, 24, The Wire- can all be traced directly back to the likes of Big Brother for these reasons.
In response to the OP's later post, #22
Stuff like The Amazing Race are only really regular gameshows- but because they take weeks to play rather than an hour or so, aren't in a studio full of flashing lights and involve something that isn't just answering questions, people see it and think "another reality show", and gets condemned for the reasons listed above.
The Amazing Race, as a show, has been done before, a million times before reality TV got big- but now it's apparently "bad" because Big Brother happened.