Let's be honest here. More screenplays are written in a week than there are movies released in a year, probably a lot more. Even in the unlikely event that a given amateur screenplay is good, the odds of it being made are close enough to nil as to make no difference.
Obviously, that fact has some negative impact on the aspiring screenwriter. You aren't going to get rich at this. You aren't going to get famous. You aren't going to get to meet Angelina Jolie.
But there is an upside. You can focus on making the script good, instead of trying (vainly) to make it salable or produceable. You can create art, rather than just commerce.
While your work may never be seen by the audience to which it is, perhaps, entitled, the same can be said of many artists more talented than you. Van Gogh was ignored; Monet was mocked; Mozart starved; Tolstoy was forgotten; Wilde went to jail; Poe died in a gutter. By that standard, the fact that little old you aren't getting optioned by Jim Cameron doesn't really matter much.
So why do it? Sure it's fun, but not that much fun. There's fantasy fulfillment, but really that's what porn and American Idol are for.
Leo the Lion knows the right answer: ars gratia artis, art justifies itself. If you cannot create a screenplay simply because it is beautiful, you cannot create one at all.
I saw a video of a group of monks who spent several days creating a mandala for a fund-raiser in South California. While they took a brief break, an unsupervised toddler slipped into the roped-off work area and stomped around, obliterating the exquisite sand-painting in a few seconds. One of the monks, interviewed by the local news, just smiled and said, "We don't mind."
Of course. They could have carved the design into basalt: someday it would have been destroyed. The important thing was that, for an hour or a millennium, something of beauty existed.