Amplifiers designed for high end speakers.

Why isn't there just one perfect way to cook a steak?

Amplifier design and loudspeaker design are two separate disciplines. A great speaker designer may not have knowledge or interest in designing electronics, and vice versa. Also, if there were substantial “demand” for a pre-matched speaker+amp combo, there would be a matching “supply”. Finally, who’s to judge that it is a matched? A speaker can pair well with many brands of electronics. 

"why high-end speakers aren’t designed with matched amplifiers"

As @celtic66 alluded to above, a number of  manufacturers have offered active speakers with amps built in or at least deliberately matched.  Presumably, the designers believe they sound good.  Such configurations have pros and cons that have been debated on other threads.  Stepping away from the active speaker discussion, your question seems to relate more to how separates sound,

Often high end speakers are "voiced" using specific amps, and we often see those pairings at shows.  It has been said that Wilsons are voiced with Krell or Mark Levinson; high efficiency Klipsch horns are best with warmer tubes like 300B designs; Magnepans with Audio Research and Bel Canto amps; Thiels with Krell/Bryston/Pass amps; etc.

For those of us who don’t go to shows, the major high end magazines typically describe the rooms of the better-known manufacturers and the equipment they chose to demo with.  That can be a good starting point.

Doesn’t MBL make  separate amps for their speakers ? 

Vandersteen does this exact thing.