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.
How your child feels towards you is a reflection of how you treat them. Children aren’t born hating their parents.
And before some suddenly victimized unthanked employees lament about their teenage children’s dirty rooms, maybe try to understand their perspective where they’re overwhelmed with assignments and grades while also being pressured to work and own property as early as possible, while being bullied by other teens for being unemployed and not a homeowner at 14 and dealing with fake friends and crap on top of all the usual teenage emotions and hormones and crap, on top of you antagonizing an overwhelmed human child for showing signs of being overwhelmed.
Loved children aren’t filthy and depressed enough for their living space to suffer. Buying iPads isn’t love.
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.