Because it puts the onus on me to sort out a plan. I have one particularly friend who does this frequently. She lives about 40 minutes away in a place that i' m never just passing. She definitely doesn't spend her whole weekend sitting in the house, so if I did just drop by she may not be in. And she spends a lot of time in her huge garden, without her phone and where she can't hear the doorbell, so again if I did just drop by I might end up having to climb over her back fence to get her attention. So what actually happens is I end up having to do all the work to arrange a convenient time to go round.