Why monoblocks?

There are many benefits to using mono amplifiers. By having completely independent amplifier channels you have zero IM distortion between channels, no crosstalk. Each amp has completely independent power supplies, not shared from a single transformer, allowing for greater dynamic range. Mono amplifiers are able to be built with larger cooling fin sections on a single chassis that allow higher power outputs than typical stereo amps are able to produce. Of course, there's the visual impact of two big amps in a system making a statement that owners of big systems take pride in displaying. Mono amps are cool.

"Most amps can handle most speakers"? How well they do it is another matter all together.

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.