You could just as well say "why do well known songs get sung so fast", because that happens too - in both cases, it all depends on the singers and the songs.

But singing a song that people normally gallop through without thinking about it, and doing it at a pace where the words actually get listened to - that can be worth doing sometimes.

For example, I remember once hearing the Rocky Raod to Dublin sung slowly, and it brought it out that it isn't just a jolly romp, there's a level of meaning in which it's pretty heavy stuff about emigration and what it involves, and what it does to people. Not the only level of meaning, but one that is lurking there, below the fun stuff.