Because:
A) you have to spend 10mins zipping yourself up in a leather suit every time you go out.
Granted it does take longer to get ready, however, you save 10minutes easily by not being stuck in traffic.
B) you're exposed to the elements. So you're either sweaty and gross or wet and freezing.
If you buy the right kit you'll be fine. Again I accept in extreme conditions you'll never be 100% comfortable, but you'd be surprised how well you can keep the elements out
C) you're at risk of being squashed by cars, tumbling into fences, slipping over on manhole covers, potholes etc.
True, but there are risks in cars, they are just different. If you are observant and practice good roadcraft you can anticipate better, and miss potholes and so on, yes it is less forgiving on a bike, but at the same time you have as much chance as it not happening anyway if you're careful.
D) you have to be concentrating at all times to maintain your balance.
I take it you don't ride a bicycle? You don't need to concentrate on balance - concentrate on the road though, absolutely, as you should be doing in a car anyway.
E) no stereo.
You can get ear pieces to listen to music.
F) no storage space, you can't buy anything while you're out.
You can improvise, or buy a box - granted large items you cannot collect - but then you don't use a bike for those things. I can honestly say that I've never gone out in a car and suddenly bought myself some flat packed furniture on a whim....
G) even having one passenger is a complete pain.
Well you can have 1 pillion, and it's not a pain, but it is different. Besides I've generally had 2 seater cars - so it doesn't bother me
H) ruins your hair by the time you get to your destination.
I'm not vain so I don't care. But I also have little hair.
I) most sports cars are plenty fast enough for most roads. Unsure why anyone needs such straight line acceleration that a bike has.
Because they're dirt cheap and will be faster than most supercars. But it's not just about all out acceleration, you form part of the bike, in my case, I am at leat 50% of the weight of the bike, so my interaction with how the bike behaves is down to me. You are almost "at one" with the bike - it's a far more involved ride/drive. There's nothing like nailing a corner on a bike and getting it "right".
You either "get it" or you don't, but a bike ride is a far more symbiotic relationship and experience.
J) they all sound like hair driers compared to any larger engined car.
Scooters yes - bikes? Errr no.