Not necessarily lazy or stupid
As absurd as it looks in retrospect, there was a lot of serious debate around the decision to go Flash back when it was made in 2010/2011 and the answer was not obvious. The Web was just a whole lot less capable back then. But the major sticking point was a web client would have only been able to deal with vCenter and not individual lonely ESX boxes. A middle tier would have been essential in those days and they were unwilling to put one on ESXi, which had generated quite a bit of its own controversy around that time as they shoved that down everyone's throats.
I don't keep tabs on it these days but it's not clear they ever solved the problem of managing just 1 box.