See! I told you that sometimes you'd get the why.
Reason #1: Why playing piano is better than playing flute
One word: CHORDS! After 25 years or so of playing the flute, it is absolutely delicious to be able to play more than one note at a time. Flute is nice - don't get me wrong - but when you don't have an ensemble to play with, your sonic adventures are quite limited. But with the piano, you don't need those pesky clarinets, trombones, or French horns to sound good. (Well, French horns are always a welcome addition to anything musical.)
Often I'll just sit at the piano and play with different sounds until I almost fall over with delight. Tension, tension, tension, aaaaaaaaand...release. I'm grinning just thinking about it. The theory of it all rather escapes me for now (major! minor! 7th! diminished 7th! minor major 7th!) (oooh, check out this page if you want to make your mind swim with all the different chord possibilities!) but I like experimenting with the different clashes and compliments.
Reason #2: Why playing flute is better than playing piano
Sometimes, I just want to not suck. Let's be totally honest here - playing the piano is hard! There are times when I'm expected to do a split-second glance at the page and then play EIGHT NOTES AT ONCE. While the result may sound pretty fantastic (see above), it takes almost all of my brain power and waaaaay longer than one-tenth of a second to do so. And what the hell is with bass clef? My god, what sort of evil, wrinkly soul thought this up? I spend 25 years of my life with Every Good Boy Deserves Fudge as my musical mantra and then all of a sudden I have to change it to Good Boys Deserve Fudge Also? Oh and don't even get me started on how FACE becomes All Cows Eat Grass. (Though, according to this page I should be thankful I only have to worry about 2 clefs at a time!) Anyway, you get the idea. Playing piano = tough.
Playing the flute, however, is something I can do on autopilot. When I see a C on the page, I don't have to think, "Okay, what note is that? F-A-C - C! It's a C! Okay, what fingers do I use to play a C? Um...oh right! Index finger on my left hand, pinky finger on my right hand and...that's it." I may have done that when I was seven or eight but now I just see that blob that means C and my fingers automatically go there. Mind you, I'm not as good as I used to be but I definitely don't have to think the whole time I'm playing. If I'm playing a song that I used to play with the concert band I belonged to, my mind will fill in the other instruments so that, in my head at least, it sounds like I'm playing with an entire band again. Or maybe I'll start thinking about other times and places where I performed the song. Or maybe I won't think at all and I'll just listen and feel.
Reason #3: Why singing kicks both their asses
Singing feels great! When I'm playing the flute, I'll often adopt a hunched-over, almost fetal, position that I imagine makes me look super cool (but would probably make this guy red with anger). After an hour of playing, there are unpleasant twinges in my back and an alarming number of fingers have fallen asleep. Similarly, after a good solid go at the piano, my back, butt, AND hands all ache with equal urgency.
But singing? Singing I can do any time, any where with great ease and no physical discomfort. Whether I'm doing dishes, cooking supper, having a shower, or - when I'm feeling particularly reckless - walking down the street, nothing feels better than belting out a song I love. The way each particular note sneaks up and out from my lungs and makes my whole body hum with acoustic resonance never ceases to lift my mood.
Ah music. If you'll excuse me, I think I need to go...sing. No, play piano. Or flute. Um, maybe I'll just go to bed.