If I might add something... part of the perception that barely anyone gets banned comes from few main aspects:
1) Most of reports are simply false (which is connected to what Ragnarok said - most of 'cheaters' either are better players or bullet sponge because of connection problems)
2) Fairfight requires a certain sample size in order to identify someone using cheats - it will not magically detect a cheater before he even starts playing. It really does a decent job in combination with PB, but it needs some info in order to work.
3) If you report someone, you will NOT receive the results of investigation. Also, that investigation tends to take a while due to sheer number of reports.
4) If someone gets a full ban, you will not see a huge 'BANNED' writing on his profile - from outsider's standpoint, his stats will simply stay the way they were, but he will not play. At all.
So, in conclusion - it's a mix of false reports, anti-cheat programs needing time to react (which is the way anti-cheat programs work in EVERY online game) and you not getting any visible info regarding whether someone got banned after the report..