In a lot of ways the cleverness, cynicism and strangeness of the Marx Brothers suit modern tastes. Many people say they're their favourite pre-war film comedians - but I always suspect they are isolating individual moments and not watching the films in their entirety. Because as films, they are rubbish. They really fall between two stools - feeling obliged to provide some kind of tedious romantic plot, but not even attempting to integrate it with the comic routines. As a result these are as random and pointless as the slapstick skits in a pantomime. Groucho's wordplay, Chico's organ-grinder schtick, Harpo's general weirdness and Zeppo's...whatever he does*: they enter, they exit, on to the next unconnected bit of business. Then an arbour scene between the romantic leads, followed by some second rate cast 'number' and a dance routine.
Now, don't get me wrong: some of the Brothers' bits are very good, but it's not enough. The best films, like Duck Soup and Horse Feathers, do have a degree of coherence; but great comedy also needs meaning - not a message, but to have truth behind it - and it needs a heart. The Marxes, as characters, have no heart; they're like demons. Madness in their eyes, they prey mercilessly on everyone around them - until the denouement requires a bout of schmaltzy sentimentality to make sure the audience don't go off and have nightmares.
I don't know why this set doesn't include Day at the Races (my favourite) or Night at the Opera - but obviously without them it can't be seen as remotely definitive.
*To be fair, Zeppo was a pretty good singer; but that's not exactly what the act was most in need of.