All movies are entertainment. It just depends on who is being entertained.
So too with fiction. All fiction is entertainment. H G Wells, it is remembered, believed that the purpose of fiction was to educate, it had to have didactic aims; but he never lost sight of the need to entertain first and foremost. A little bit of sugar makes the medicine go down.
Those novels which have survived are, for the most part entertaining to a sufficient readership to ensure that survival. I personally am not entertained by Joyce's Ulysses, but thousands are. I am entertained by the original Ian Fleming James Bond novels, but thousands--millions--aren't. But enough are to keep them in print nearly 50 years after Fleming died. (This year is the 60th birthday of Casino Royale.)
Best-sellers come in two main types: the big gaudy bubbles, which burst and are never heard of again (think Peyton Place, By Love Possessed, The Green Hat); and the ones that never seem to go away (Think Tolkein, Fleming, and probably The Godfather...)
Unfortunately, in the world at large, very few people actually read novels at all. A huge best seller like, say, the Godfather, sold a few million copies; but there are 250 million people in the US, 55 million in the UK, and another 100 million or more native English speakers about the place... so that blockbuster reached maybe 2 percent of the available population. Very few novels no matter what their merits or demerits, reach one percent, and most authors would be very happy with one tenth of one percent.
I read Dickens' Great Expectations and Hiassen's Skinny Dip for the same reason: entertainment.
Re-reading this, it doesn't make a lot of sense, but WTH.