Maybe they are worried about the poor inexperienced piano player trying to keep up. 
I’m sure that is sometimes the case, but not always.
Personally I have told my children the songs they are to sing at my funeral, and have told them that if they sing them too slowly (they are up tempo songs) that I will personally arise from my casket and lead them at the proper tempo myself.
I have listened to songs at funeral tempo my whole life, so at my funeral we will sing songs at the correct tempo.
😆 I hope you’ll forgive me then if I am reluctant to attend. As much as I would love to hear songs sung at their proper place, I’m not that eager to be led in them by a corpse!