My initial feeling is that the most crud left behind is from professional fakers, aka menial labor from India/Phils/etc. That's where one can buy fake reviews en masse.

That explains point 1, 2, 3, 5 and possibly even point 4. (working time difference, and cheap equipment.)

The key to detecting fake accounts is tractability. That's why FB logins are gold. You can look at the account and - most of the time - it is easy to tell if it's real or fake.

The problem is that many sites want volume, not quality. They just let anyone "add content". This is a an easily solvable problem that most sites do not actually care to solve.