I've actually encountered relatively few adventures where there was "too much" prose. Instead the issue I see is typically misplaced prose and/or just goddamn terrible organisation to the point where I practically have to re-write the thing to make it usable. By misplaced prose I'd particularly mean prose where they basically tell a a story in a linear way, rather than y'know, providing the information to run the adventure. Like, for god's sake sum up the ENTIRE adventure briefly and precisely at the beginning. You should not need more than a few paragraphs. Don't give me some bloody flavour text and make me read the whole thing to work out what's going on. I'm the DM, not a player. I know I'm expected to read the whole thing, but that's a hell of a lot easier to make sense of if you know what's supposed to be happening.
That does apply to NPC descriptions, like the OP was suggesting - you often find NPCs who have their entire, totally irrelevant backstory or overdetailed personality written down, like they were someone's dodgy PC, but at-a-glance info for RPing them isn't present (requiring you to make notes) - and this happens with NPCs who there's a large chance the PCs won't even speak to in some cases. One adventure I played in broke down into farce because the DM himself was so bemused with the ludicrously overdescribed NPCs and had them unnecessarily yelling their backstory at us and stuff. It was pretty funny, but it illustrated the problem. Some random cooks all had multiple paragraphs of detailed backstory. This seems to be fairly common.
I feel like this is partly audience-driven. I know an awful lot of people buy adventures and don't run them (sometimes buying them for games they don't even have groups for), and those people, in my experience, do prefer "read it like a story" adventures, but I wish they were just, a different market. I have no time for that. Write adventures so they can be run, not read. It's like designing a car to look at, not to drive. It's happened - a lot - but it shouldn't happen.
With rulebooks, prose is a much larger problem.
Countless rulebooks have acres of what is usually mediocre or outright bad fiction in them. There are times when this is useful. If the setting is highly unusual, or has a weird vibe that you need fiction to convey, okay. But that's almost never the case, instead you have utterly prosaic settings and rulebooks jammed with tedious prose. I haven't played recent Fading Suns but there was a ton of this in the older editions, indeed, it was extremely common in a lot of '90s RPGs (looking at you White Wolf). I don't want to read tedious fan-fiction, guys. If it's compelling and/or describes something special about the setting, great - but again, that's the exception, not the rule.
On top of that you have rulebooks which don't do that, but which do insist on burying rules within non-fiction-based prose, taking a highly descriptive approach to their rules. Worlds Without Number, sadly, is a good example. I love WWN rules-wise, but why bury important rules half-way through a paragraph that isn't even really about that rule and which occurs several paragraphs after the last previous rule. I'm pretty sure a WWN "Just the rules, ma'am" would be like under 20 pages and also I'd probably find out a couple of rules I didn't even know about.