Bubble effect, people that play inside a bubble while avoiding everything else tend to have this issue that do not know/understand the actual metrics.
Either way, the OP isnt as terrible, averages 55-60% logs, compared to WoW population he is still in the "Better player" category, blasting him is just bad manners.
To the actual problem OP, start your own group, problem solved, thats how the pug world is.
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Sin isnt bad, Sin is bad in the 99 percentile and if you are raiding Mythic competitively, and thats already over, its almost the third month into the expansion, a decent player that knows how to play Sin (and its an easy spec) will do more than play the FOTM spec at anything under 75 percentile, people need to understand this