You could ask the same question about music or painting... Why listen to Beethoven when you've got Taylor Swift?
Part of the answer to your question is that a movie has to have been around awhile so that any initial enthusiasm has been replaced by a more reflective assessment as to its value. In other words, "instant classic" is an oxymoron. Another part of the answer is that what people consider to be classics change over time as well, for example, "Citizen Kane" was hardly mentioned on "Greatest Movies" lists until the late 1950's but recently has been supplanted by "Vertigo", which also hardly appeared on anyone's list until the late 1970's. So we keep re-evaluating these older movies. Doesn't mean though that the latest round of evaluations is more "correct" than the last one. But as to why a movie that is only three years old, let's say, isn't on those lists generally is simply the mistrust of initial impulses, the impulse to say, for example, that "Avatar" is the greatest movie of all time, only to find its reputation to have dropped a bit over time. I remember people saying the "Dances with Wolves" was one of the greatest movies of all time when it was released. Nothing wrong with those movies but honestly, how can anyone say anything is that great when it's brand-new? It takes TIME to evaluate something's worth more accurately.
Another factor is just generational. A "great" director today is often in their 50's, let's say, which means they were a teenager in the late 1970's, so the "great" directors that they studied in college in the 1980's, when most people get the most passionate about a topic, were in their later years by that time, which means their heyday was the 1950's, 60's, etc. -- i.e. Lean, Kubrick, Hitchcock. A bit younger, and maybe your heroes would be from the 1970's -- Altman, Coppola, Bertolucci, Scorsese, Spielberg. I've met people younger than me who are big fans of John Hughes' 80's movies, something I never got into, just like I was already too old to get into 1980's pop music like Michael Jackson (I graduated college in 1984.)
Sure, we can argue about what is "wrong" with modern movies but then again, it's probably too early to assess them properly.
Also, there were plenty of bad movies released in the past, but over time, we have been able to see the cream rising to the top, let's say. With contemporary movies, we're still in the middle of it all so it's harder to know what will stand the test of time.
Commercial impulses pretty much mean that current fads are always at the forefront, so I don't see a problem with taking a moment to reflect on the value of older movies -- we see plenty of stuff being shot in completely contemporary styles because that's commercially successful. Contemporary movies don't need to be "championed" over older movies, the pressure is always to be contemporary after all.