people using the same 'mons and same sets largely has to do with the fact that those common things are proven to be easily effective, and the majority of players not really having the skill to be able to make use of less conventional things, especially when not all 'mons are created equal and most battlers are more interested in winning (which is normal)
a fair bit of what's on Battle Spot is probably hacked, at least in the sense that they were not necessarily generated and obtained via 100% legitimate means. but almost all competitive battlers couldn't care less if the 'mon was genned or not, just so long as it's stats/moves/EVs/etc are ones that it is at least theoretically possible to have, regardless of how (incredibly) improbable it is. years of people SRing and breeding and catching shit (with these things being made easier over the years) makes full shiny teams and legendaries with the perfect IVs more and more probable, especially when you factor in cloning and WiFi trading
every case I've ever personally seen of "why did this situation happen, must be hacked" ultimately ending up where, after gaining more information about what happened, it was found that there was no hacking involved and that what happened was indeed possible