Just for the sake of repetition, I'll reiterate a few things, being someone who has driven both a CVT and a 6MT AC.
The CVT is neat. It's a big advancement with respect to transmissions. Nissan is the only company who is somewhat successful with their implementation of CVT transmissions into their lineup. They are better than MTs for daily commuting, especially in heavy traffic, simply because they enable much better fuel efficiency. On a long term scale, because they don't work like conventional transmissions, they will allow engines to last longer because there is barely any fluctuation in the RPMs when driven under normal conditions (and not in manual mode like a racecar

, allowing for less wear and tear on engine internals.
In addition - Nissan has chosen a different target market for these cars than say a Z, and as such, very few Altimas (both coupes and sedans) are manufactured with 6MTs. Actually, the production numbers (for 2010 models) say 3% of ALL Altimas produced (sedans and coupes) are in the configuration of V6 CPE 6MT. Sedans, as of 2010, are no longer offered with MTs for any trim at all (I have a source for this, but I'm lazy right now). This makes for a very rare configuration in the grand scheme of things. Some people don't feel like hoofing it all over the country to find one. Most people don't feel like paying the premium that you'd pay in ordering one from the factory through a dealer.
I have one because I lucked out and found one. I have owned two CVT models prior to this one. I took a bit of a hit getting this one, but I don't regret it and know this is a vehicle I will own long term.
Most people hardcore set on doing things like turboing, etc, are going to opt for the Z. But the Z is no longer as accessible to some people that end up seated in Altimas because of its price. A loaded V6 Altima coupe lands you in entry-level Z pricing. The Altima I'm driving MSRPs just south of the $32,000 mark (kick plates, floor mats, leather, premium pkgs). To have the same amenities in a Z (bluetooth, bose, ipod interface - read NOT aux jack, actual interface), it lands you just south of $42,000. Some people would rather spend $10k on wheels, suspension, exhaust, and a turbo. Some would rather have a stock Z.
Keep in mind also, a lot of ppl on here bought cars with CVTs before they knew about any of the performance issues you've conveniently waltzed in to find out about without having to experiment for yourself. It took guys in this thread to pave the way and uncover those issues. Had they known before they spent thousands of dollars modding, they might not have. But then no one would be able to make educated decisions about it. That's what these forums are for. In theory, the CVT is better than a manual because it doesn't disrupt the momentum of acceleration with shift shock that you get in a manual. If it could only hold the power, no one would drive a manual. The CVT is just not there yet.
So before you ask the question of "why not a manual you morons?!", keep in mind a few things:
1) to each his own.
2) manuals are soon to be a thing of the past. (see NISSAN GT-R, the automatic that shifts faster than any human ever could with a manual)
3) you'll get long-winded posts like this one.
4) some people might be missing appendages or something but don't advertise it. (I have a friend with half of a right arm. he might prefer a CVT over a manual, and he prob should still be allowed to do whatever the hell he wants with his car).
5) Brett Favre.