Because the voices in their heads tell them to… 
Seriously, there are any number of reasons that flight times and numbers change. First, flight times…
The first thing you have to realize is that while you are concerned with getting from point A to point B, the airline is concerned with getting the aircraft itself from A to B to C to D to E then back to A. Meanwhile, the other passengers on the plane are going from A to B to J or from X to B to C or whatever.
Suppose, for example, that lots of passengers on the B to C leg are originating in X, but the flight from X to B tends to be delayed a lot or that it lands on time but there are so many passengers that they cannot process all of them before the scheduled departure time of the B to C flight. The airline may adjust the times on the X to B or B to C flight in order to avoid the problem. But, since the entire network is interconnected, adjusting the time on one flight may cause new problems on a different flight. The ripple effect of changing one connect time could affect dozens of flights.
Also, airlines are assigned certain gates and times by the airport authority. Adjusting the time on a flight may mean there is no gate available for the flight when it lands or no departure slot for it when it is ready to leave. Again, changing one flight may affect a number of others.
While flight times tend to change in response to operational issues, flight numbers tend to change in response to marketing issues.
Mostly flight numbers affect how flights are displayed on a reservation screen and how passengers book the flight. The closer to the top of the display that the flight appears the more likely it is that the passenger will book on that flight. So, flight numbers are chosen to try to get a flight to appear as high on the display as possible.
Suppose someone is flying from A to B to C and has two choices as to how to fly. One is flight 100 from A to B and flight 200 from B to C and the other is flight 500 from A to B to C.
As I understand it, the routing from A to B to C where the flight number does not change will appear higher on the reservation display than the option where it changes. So, flight numbers may change depending on where the majority of the passenger traffic is flowing.
Of course, if this involves rerouting aircraft then the flight time issues appear again.
Also, lower flight numbers tend to appear closer to the top of the display so flight numbers are adjusted depending on what the other flight numbers in the market are.
As a general rule, traffic in the summer months is different than traffic in the winter months, so flight numbers and times tend to experience major shifts around twice a year. Minor changes can occur at any time, however, depending on changes in local conditions.