I think you might be parsing the sentiment of the phrase a bit differently than how it's intended.
It sounds like you're interpreting it to mean "parenting sucks because my children hate me and never listen".
I would hear the phrase as closer to "parenting is difficult because the person the work is done for variously doesn't see the work being done, takes it as a given, or only expresses any gratitude far, far down the line because as you said: "The reward from parenting is raising an awesome strong kind productive member of society that you could actually be proud of and create amazing memories with".
It's thankless because the hardwork is often unnoticed or taken for granted, not because it's difficult.
People should keep in mind what you say about teenagers, but I don't think it's related to the phrase.