First off, I want to point out that not all movies can suck. A movie has to try to be good but fail before it can legally suck. Or your expectations must be high enough that its failure makes it suck. So sucking is relative in many cases. Does the remake of Planet of the Apes suck? No, because I never expected it to be good. Tim Burton's Sleepy Hollow sucked so successfully that I never imagined the monkey movie could be great.

Why would someone spend time making a movie only to have it suck?

Some filmmakers don't care. Some care, but aren't very smart. Some lack the resources; some drown in an excess.

Just remember that what we bring to a movie, our experience, our intelligence, is just as important as what the filmmakers brought. The ability to understand why a movie does or doesn't suck, to watch a windmill or helicoptor explode for no apparent reason and have it bother you, to spot a plot twist forty minutes early, to not laugh when the old lady flips someone the bird, and to never like a movie solely because the critics liked it, is one gift only those who have can fully appreciate.

Don't let anyone treat you like an idiot then tell you, you were entertained.