Because it's generally funnier to the organism when more senses are involved. Hearing a joke can evoke laughter, but witnessing a perfect delivery of that same joke is more likely to reach deeper, into the "laughter reservoirs".
So, yeah, that's why :)
Yes...just...yes...
Get's up slowly, whilst looking deeply into @steemonme 's eyes
Starts a slow clap
Licks right corner of lip, with a seductive look in the eyes...
"Bravo....bravo..."
Nice
hahaa
Never mind me. I was drunk when I wrote the above. =D