I came across the following intro to Avant-Garde Film Studies by
Scott MacDonald.
His points are obvious to many of us, but he hits the nail right
on the head.

'Mainstream cinema is so fundamental a part of our public and
private experiences, that even when filmmakers produce and exhibit
alternative cinematic forms,that dominant cinema is implied by the
alternatives. If one considers what has come to be called avant-
garde film from the point of view of the audience, one confronts an
obvious fact. No-one or certainly,almost no one-sees avant garde
films without first having seen mass-market commercial films.In
fact,by the time most people see their first avant-garde film, they
have already seen hundreds of films in commercial theatres, and
their sense of what a movie IS has been almost indelibly imprinted
in their conscious and unconcious minds by their training as
children and by the continual reconfirmation of this training during
adolescence and adulthood. The earliest most people come in contact
with an avant-garde film of any type is probably mid to late teens
(for many people the expereince comes later, if at all.
The result is that whatever particular manipulations of
imagery,sound, and time define these first avant-garde film
experiences as alternatives to the commercial cinema are
recognizable only because of the conventioanlized context vieweres
have already developed.
Generally, the first response generated by an avant-garde film is,
" This isn't a movie," or the more combative, " You call this a
movie?" Even the rare, resoponsive viewer almost inevitably finds
the film-whatever its actual length in minutes- " too long".By the
time we see our first avant-garde film we think we know what movies
are, we recognize what " everyone" agrees they should be; and we see
the new cinematic failures-to-conform as presumptious refusals to
use the cinematic space (theatre, vcr, viewing room)" correctly".
If we look carefully at this response, however we recognize that the
obvious anger and frustration are a function of the fact that those
films confront us with the necessity of redfining an expereince we
were sure we understood.We may feel we KNOW that these avant-garde
films are not movies, but what are they? We see them in a theatre;
they're projected by movie projectors,just as conventional movies
are...we can see that they ARE movies, even if we KNOW they're not.
The expreience provides us with the opportunity to come to a
clearer,more complete understanding of what the cinematic experience
actually can be, and what-for all the pleasure and inspiration it
may give us-the conventional movie experience is NOT'.

yup, that sums it up quite well