If PCs have spell lists, monsters have spell lists.
This is a colossal practical problem for most groups. One person controls a PC, and that person has controlled that PC from the beginning and knows its abilities inside and out. a DM has to control multiple monsters as well as run everything else in the game, and frequently he's using a given monster for only the first or second time. In other words, there is a clear, huge, practical reasons why PCs have spells lists and monsters must not.
And, while looking up things is annoying, having content duplicated just so it can be in a monster stat block is really annoying, because it means I'm paying money for the same content multiple times. It also tends to make stat blocks unwieldy. I hate when monsters go over 1 page. Hopefully, online tools can make referencing easier, but there really no better ways of doing this.
Definitely, having content duplicated is annoying. There is a constant trade-off between duplicating content and not including everything in a place where it is a useful reference. The only question is where the balance lies. I would say with monsters with complex, detailed spells and abilities, the balance easily lies on the side of needing it as a reference within the monster stat block. DM prep time and combat length are
massively increased otherwise.
I agree monsters going over one page is a pain. I don't think they'll have to with a concise 4e-style statblock, even with added environmental and lore information.
Incidentally, does this mean that people are actually using stat blocks straight out of the monster manual during play? Monster books are usually my facorite books. However, in over ten years, I've only done this a few times, usually for summoned monsters and always out of desperation. Regardless of how it's formatted, the monster in the manual is an 'average' monster, something I would never want my PCs to encounter. Thus, I'm not really seeing how usability of a stat block in play is a big issue.
Yes, definitely, many (most?) DM regularly use stat blocks right out of the book. Reworking every stat block takes time, lots of time, even if the system facilitates it well. And sure the monsters are average for the fantasy world of the game, but are they "average" for the players (i.e., have the players seen that particular stat block before many times, so it's old hat and boring)? That's unlikely; to the players even the average monster (in the game world) is still interesting.
I'm normally a bit less direct when I post on here, but this is too important a practical issue to hedge on. It is absolutely a dealbreaker for tons of DMs. If I'd have to manually look up half a dozen spells during combat (or in prep for a game) just to run an interesting magical monster, I would not run that system. It's just too much of a colossal pain. A one or two line summary of every spell-like ability a monster has is an absolute must for a usable monster manual. This is one area where 4e was a massive, super-practical, game-changing improvement. I'm not going back.