As an old-time player I am familiar with the phenomenon ya'll are discussing. Here's my 2¢. In a performance situation like a concert or a dance, my band and I make sure that we switch keys frequently, as well as switching tempos time signatures, mixing up songs and fiddle tunes with waltzes and rags. Our banjo player is fast enough at re-tuning his instrument that we don't have an inordinate amount of time waiting for him.
However, in a jam session setting where the object is to spend time, socialize and share tunes and not to entertain, we will tend to stay in a single key for longer periods of time and maybe even for a whole evening. There are a couple of reasons for this. First and foremost is that in old-time music, we are striving for a specific sound. That sound is achieved by using certain specific tunings on the banjo and fiddle. Sure, a fiddler can play Tommy Jarrell's "Boll Weevil" in standard tuning. But in order to get the droning sound that I'm looking for, I have to tune to DDAD. It just doesn't sound right if the low string is tuned to G. I just can't get that low drone that I want. Same for the banjo player; a bluegrass banjoist can play just about anything out of open G tuning, but it doesn't sound the same as a banjo played clawhammer style in aDGAD tuning playing "Sally In the Garden". If we play in a given key for long periods of time, it keeps us from having to wait while the fiddle, banjo, or lap dulcimer re-tunes.
Additionally, most good old-time players know enough tunes in the keys of D, G, A and C that we can play for several hours in a single key and not repeat a tune. And since we're not playing for paying customers but for ourselves and we're keeping ourselves amused, who cares if we change keys?