Of course some writers are sincere. But the more popular a writer is, the less artistically sincere we should expect that writer to be, because business concerns are liable to interfere with the artistic endeavour. There are exceptions, but that's a handy rule of thumb, I think.

And this is especially so when the writer's platform is biased against certain genres or when the readership is mostly insincere. When most readers want to be entertained rather than informed, and yet they come to read, chances are many of these readers are fooling themselves. And if readers want to be fooled, savvy writers will oblige and fool them. Are those writers being "sincere"? They'd be honest businessmen but dishonest artists or philosophers (truth-tellers).