Because people believe the schedules even when they are unrealistic.

Schedules are usually made with the expectation of good weather; when there is anything other than good weather, delays occur. Notorious examples are SFO and ORD.

If possible, choose non-stop flights, so that you do not get stranded at a connection airport. If a connection is unavoidable, choose connection airports that are less delayed by weather problems, and choose layover times according to the origin and connection airports' risks of delays (i.e. choose a longer layover if you are originating or connecting at a high risk airport like SFO or ORD).