When you call someone out for cheating; why do people always assume your lying?

I have had a real problem with this lately. In my hundreds of games, I have only accused a handful of opponents who were being very blatant about it!

IE:

• A top lane who gained a large burst of health twice during the early game who DID NOT have heal.
• A who was the most obvious scripter I'd ever seen! Even attacking a stealthed with Auto attacks several times with no Vision wards in sight. (Also no, the enemy team didn't have . )
• And Just last night, a mid lane who had impossibly high damage!

I was playing as with full tank Masteries, and Full Armour / MR runes, At Level 3 Her auto attacks were dealing 320 damage, what's more by lvl 7 she had build full AD, rushing and ; and her E dealt more then 40% damage to me!

I looked at her build and Match history after the game, Her runes were standard ADC and 10 out of her last 12 matches had her dying 12+ times with 4 kills tops.

When I tried to tell my team mates during and after the game, all I got was " OMG Shut up noob cho!", or "Cry some more you feeder!", "Your just getting rekt! Shut up!", ect...

It doesn't happen a lot, but people do hack and script in this game. The part I don't understand is why so many people just instantly assume your lying if you bring it up? I've only had 2 people on my own teams ever accuse an opponent of hacks / scripting, so it doesn't seem like the sort of thing that happens to often. Is this some community taboo I'm not familiar with? Or do people just truly believe it never happens?