Mic, mic…where is the mic? How do they hide the microphone on actors? You don’t see it pinned to their lapels, for one. And, if hidden under clothing, why don’t you hear the rustle of cloth across the mic, esp when they hug, for example?
OK, so how do they reduce the echos? Shadows of the boom(s), or multiple shadows? Why do all voices sound of equal strength? A stage is not an anechoic chamber.
Boom mike operators are very good. They avoid shadows and place the mics as close as possible, constantly adjusting as the actors move around. A stage is not an echo chamber, either.
You often do see it pinned to their lapels, actually. Or on their tie, or on any large frilly thing near a woman’s neck. And mics have gotten smaller in the last ten years, so they’re harder to spot.
Well, I haven’t seen them on television shows, but there are several movies out there with boom mikes visible.
Maybe the editing is better for television?
Yeah don’t forget that most sitcoms are filmed on a sound stage. An office scene is not filmed in an actual office. The walls are completely different, the “ceiling” is very controlled and mics are very strategically placed. As are actors.
Shows that have outdoor scenes, say like Law & Order, put boom mics to good use. Also, if there are any sound problems, scenes can be over-dubbed.
The production of Survivor is amazing. In all the time I have watched survivor, I seldom see the crew or equipment or even the shadows of the crew and equipment. In fact, the only time I can remember seeing a camera man is on a family member visit when the family member went running to the castaways. I guess production lost control of the scene.
Can’t speak for TV, but live theater sometimes uses mikes taped to the actors’ chests. They’re very small, and with skin-colored tape and makeup, very hard to spot. Reduces some of the clothing noise, too.
35mm movie film has a 4:3 frame aspect ratio if used in the traditional 4-perf way. If a film is being shot in traditional widescreen (1.85:1 aspect ratio) on 35mm film, the top and bottom areas of the 4:3 frame are not used and may be matted out. Sometimes, however, the scene will be shot in full 4:3, assuming that the projectionist will correctly matte the film when it is shown. In this case, boom mikes, lights, etc., may be seen in the top of the frame. If the projectionist doesn’t matte it correctly (or the film lab screws something up) then these might be visible on the movie screen, even though you were never supposed to see them. Some films are notorious for close tolerances that caused boom mikes to be visible in a lot of shots (Thirteen Days is an example.)
On TV you don’t have to deal with a projectionist screwing up. 
Also, fewer and fewer things are shot on film for TV these days.
I’ve only noticed these in plays and talk shows.