Your post is very black and white. Life isn't like that.
The years are short with children, and its a balancing act between things they need and what brings you (and them joy).
Someome saving to take the family away for one week a year (for us 2.5k a year) isnt really going to do much for a pension pot for me, my DP or my kids is it?!
Saving £300 each child per christmas again 18 x 300 is £5400, not really going to do a lot for a pension is it?
And not doing those things robs joy out of life.
Having one family weekly holiday a year and spending some money on your children for Christmas isnt luxurious or extravagant or indulgent. Its pretty standard life. Imagine "sorry kids, no xmas presents...but im paying into your pension"
"Sorry kids no help with driving and a car but ive got you a pension pot"
People with not very much are struggling to pay for the BASICS, homes, clothes food. Those just about managing this may save a little bit for a holiday and some birthday presents.
It isnt because they DONT want to. It is because they cant.