I think people who always wear matching sets buy several pairs of the pants, maybe? Or they are ridiculously clean. I definitely don't wash my bras after a single wash. I wash my bras when I am too overcome by the stench not to, and not a moment before.

I rarely buy the matching pants because if my bra is £30 (and it often is) then I am not feeling kindly towards spending £10+ on a single pair of knickers. Also they are often thongs or otherwise unflattering on me. I do have quite a pile of pants that used to match bras that I have now sold. I am today wearing a pair of turquoise spotty pants that matched my one-time favourite bra whose underwiring burst free.