I dismiss them (musicals) as shallow and artless because they are overly manipulative. As Sir Peter Maxwell Davies said of War horse, 'it assumed emotions it had not earned'; and I think this applies to musicals too. There is no drama, only sentimentality.
And of course we cannot apply this to Madama Butterfly or any number of other operas.
What do I mean by shallow? Well, that there is no unsavoury content in musicals; when all is said and done, the audience can have a good laugh or cry, cheer and clap and go home completely unchanged; moved only on a superficial level. Contrast this with something like Cosi fan tutte; a play which on a superficial level may appear like pure frivolity, but which makes us feel very awkward and uneasy at times; it shakes our ideas about fidelity, it questions our deep-seated fears of cuckoldery and the possessive nature of romantic love. By the end, not one character comes out looking absolutely pure of heart. In that sense it makes us reflect upon ourselves.
I'm sorry, but for the most part the librettos of the great operas are far from being great literature. The best operas certainly surpass the average musical due to the merits of the music... and there are instances in which the libretto does hold up on its own (Strauss operas and Britten's come to mind). But this says nothing of the merits of the best musicals.
Need I mention why the music is so good?
Yes, the music in the best operas is great...but how many crappy operas have also been penned over the years? There is good and bad in every artistic genre. What we term as "classical music" is not inherently better than the finest work in other genre.
The music of musicals, if not inspired by catchy pop music, is in some flabby, neo-romantic vein. It mostly fail to communicate any of the stage action, except in vulgar theatrical effects. Contrast that with say, Wozzeck, and you can see that the inner turmoil, thoughts and feelings of the characters are expressed so well through the music that the words are almost unnecessary.
Again, you are comparing a single exemplary work of "classical music" with your idea of musicals as a whole. Would you have us believe that "classical music" is not just as laden with mediocre and cliche works... most of which have been thankfully forgotten?
On the whole though, recognise the musical for what it is; commercial entertainment.
Ah! And what is "classical music"? ART all set in capital letters? It seems to me that "classical music" is no less about entertainment than any other musical form.