Why monoblocks?

It is not at all difficult to design an amplifier with better than 50dB separation, even using valves, and any more than this is of no audible benefit.

Mono amps allow for short speaker cables and long interconnects in installations where the front-end electronics are located a distance away from the speakers.

Primarily prestige. A LOT of folks who show thier system clearly have a great deal of pride in the appearance of the system.
One pair of giant monobloc amplifiers shows clearly that this is one audiophile who means business.
You can see systems with TWO pairs of monoblocks biamping the speakers...
The line between real justification for a particular setup and the "bling bling" factor are sometimes blurred.
Just like the self justification for $250. outlets and $100 outlet covers, $100 each cable risers, $25,000. one meter interconnects... A pair of Monobloc amps are often just another pretty status symbol. Even if only to the owner and a few other crazy audiophiles who see pictures of them.
Sometimes it's just a 'I'm rich, so I can throw money at anything and have the best.. And to dealers.. the best is nearly always a pair or two of monobloc monsters.
If your look at the systems owned by folks of modest means, and see a pair of monoblocs.. they just are crazy audiophiles.. (me too! though I do not own monobloc amps).
So Monobloc amps are: A status symbol, a way to throw money away on audio equipment pressed for by dealers, a way for the insane audiophile to SHOW they are devoted.
If I had the money I can guarantee you I too would own a pair of monster Audio Research monoblocs, and maybe another quartet of MBL.
(8^Q...
(and for all you OWNERS of monoblocs.. feel free to flame away.. Hah hah hah hahh haaaa I am just jealous as all get out!)

Good responses so far, but not a lot from mono block advocates and what if any improvement in sound they hear going from or comparing a stereo amp to monoblock amps.

Monoblocks with long interconnects and short speaker cables versus stereo amp with short interconnects and long speaker cables resulted in more micro detail, lower noise floor, more expansive image, and generally more realistic presentation.

My monoblocks have ranged in price from $2000 - $6000 (used). Not exactly a display of wealth in the world of high end audio.