Because barring a radical revamp - which this isn't - the source material is irredeemably middlebrow and naff, and they keep singing everything* and by the time that stops being ridiculous the film's been going on for *too* long and you're numb to everything.
Also: Anne Hathaway.
* That's not to say 'Full Musicals' like this are always bad, but they either take or they don't - and this doesn't.
Preening, Regan-era yuppie brats prat about in Chicago for the day in a completely unlikeable fashion, bending everyone else to their monied will.
When they're not humiliating their family and peers or offering crumbs of their privilege to brown people, the film attempts to inject some heart and humanity: there's a sudden, largely unexplored bit where Ferris wants to marry his girlfriend; Cameron, the almost likeable third-wheel, has daddy issues; and there's a vague Coming Of Age thing. But for…