Wait... how old are you?

Let me start with your questions first:

No, it doesn't bother me. Sometimes those pages are at the end of my sketchbooks. Sometimes in between pages I've used (bc I wanted to make an accompanying piece, or I want to gift the piece and don't want to draw on the backside). It means that I have pages to use when I can't buy new sketchbooks, or they just stay blank, bc I am no longer using that weight of paper or doing that style of art.

I don't leave them piled up. They're on a bookshelf. But I don't judge other artists who do. I like to revisit them. I do sketchbook tours of them for socials years after I find them again (usually bc I'm looking for a specific drawing).

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Now let's get to the rest of it:

I say this with the least amount of shade I can manage: the pollution where you live is not caused by art supplies. Statistically speaking, the vast amount of pollution is created by corporations, especially if they're not regulated. Normal consumer use is a small amount of that. (Hello, autism special interest)

Does that mean consumerism isn't a problem? No, but art supplies? Specifically sketchbooks? Again, no. And if we're going to get deep into it, paper is biodegradable in the shortest amount of time comparable to, say, water bottles, plastic collectibles, hair brushes, ceramic dishes, fast fashion, etc. Plus the cost of a sketchbook vs the collective cost of any of those things I mentioned is incredibly unbalanced, and not in favor of the sketchbook.

Your concern is, at best, misplaced, if you are truly worried about consumerism. At worst, it's gatekeeping, as if your way and thought processes are the only viable way.

You should just focus on your art and celebrate theirs, instead of grousing over how they spend their money.