You are not wrong, it IS very cruel to leave the dog subjected to a situation that causes it distress each day.
Unfortunately the way to address separation anxiety is in the first place, to not leave the dog alone to experience it, whilst you work on behaviour modification and desensitization..
Many people, by the time they realise the dog has a problem, have commitments outside the home such as work, and don't see how they can avoid leaving the dog alone... and for those who do work it out, its expensive as hell.. daycares, sitters, walkers, professional support to address the anxiety.. it adds up to a lot, beyond the means of many..
And so they bury their heads in the sand - it's wrong, it's cruel, but I can absolutely see why it happens.
Fixing separation anxiety properly will typically take six months or more (if it was separation related problems in the first place, some cases actually aren't and are resolved faster) - that can be very daunting for some people, particularly if the annoyed neighbours expect a fix to occur in a matter of days!