I can't even imagine that they 'didn't realize'. They did, obviously, but couldn't care less. Their marketing wanted a human name so that it sounded approachable, so that's what they did.
Other voice assistants aren't as easy to pronounce as Alexa as well; Mycroft, Siri, Ok Google, Bixby.
I don't know I just like the image of MS's product being that irrelevant.
Those examples tell us that teams making these products do think about the impact of the name they choose if they're willing to accept something less "name like" will still work well, and maybe the Alexa team could have done better.
Much more common than Alexa.