Something you have to understand about Opeth is that they are a gateway band. Yeah, we knew that.
Anyway, something you have to understand about gateway bands is that, given how they're typically such earth-shattering experiences for the listener in question, the listener has probably played the fuck out of every song 100+ times (like I did). Thus, until a discerning ear is developed, the listener will have an overwhelming nostalgia so powerful that it could cause 98% of the forum posters here to disregard even the worst of songwriting.
And there's your explanation for why the posters above don't agree with you regarding Opeth's general ineptitude. Hell, the nostalgia is even so strong that I can still listen to Opeth from time to time even AFTER realizing "these songs are pretty poorly written".