I was an elder for several years, and don't remember popping in on folks unannounced except on two occasions. The two exceptions were offering condolences on a death in the family. Elders are like others in the congregation; some are considerate, some lazy, some disorganized, and others on ego trips.
Congregations develop their own personalities, too. Giving talks at various congregations opened my eyes to this phenomenon. Some congregations treated me as a welcome guest, and at the other extreme I was treated as a spy seeking dirt on the elders.
Most of us remember elders who were royal pains, but chances are we knew a few good ones, too. To answer the question of "why do elders" do what they do, it depends on the person(s) mostly, not necessarily the group in general. I agree with several of the suggestions already made; if elders call unannounced, it demonstrates a lack of courtesy at best. If the unannounced call turns into a committee meeting, it's open deceit!