To ask why do parents get softer as they get older?

Exhaustion.

I asked my mum this, because she is so much softer with my younger siblings than she ever was with me, (had me at 18 and my younger three siblings at 33 and 37).
She told me it’s more that it’s a mixture of less worrying over what others think of her parenting and more what suits her family, less worrying about the small stuff that in the end aren’t such a big deal in the grand scheme of things and also being more experienced so knowing what ‘fights’ are worth picking.

I think age gaps and thus age of parents and more parental experience time makes a difference.

Mine are close together I think there is less difference than between my siblings and me with much larger gaps. There is some with mine - mainly less enthusiasm with stuff with third - parties and school dress up days etc but we aware so try and make an effort still.

So far there isn't really more freedom mainly as eldest is by far the most sensible of our off spring.

As well as what has been said - it’s harder with more children. As some of your children get older and take on more for themselves you can relax a bit with the ones that are left. It does sound as though they were particularly strict with you though.

I'm the younger child in this situation. I think my parents were much harder on me than my big sister (she's 10 years older than me)!! They were young hippies when she was born so really chilled. They were more sharp elbowed middle class when I was young. I wonder if your siblings would feel as you do!?

OP, my siblings are 3, 8 and 9.

They're complete brats a lot of the time

I was never allowed to do half the things they do! And I wouldn't of even dreamed of doing them anyway. I was more respectful and wasn't the type to throw the words 'I hate you about' but my youngest sister does this often with no consequence.

Where as I would have been chased up the stairs and met my maker

I'm 20 now.

We mature like fine wine.,

You try to follow rules /theories about parenting with your first. You think you've cracked it when the second comes along and is different. The things that worked or were important for the first child don't work on the second who brings new and different challenges.
This is what parents of only children never face.
As you get older you realise that most of it doesn't matter anyway.

You had 8 years of 1:1 parenting. The others didn’t.

I had to get a job at 16 to afford to eat at college not up early enough to make the packed lunch offered

A combination of being worn down, and that as the youngest grow up, they find themselves with a little more time, perhaps a little more money as they've been working longer, and they share it around a bit more.

My DS was complaining this morning that his younger brother has it easier than him, because DS2 has me, DP and DS1 to look after him, but DS1 only has DP and I.

Mind you, when my littlest sister got to have a cat, and her ears pierced when she was nine, and I'd been banned until I was 12, and certainly not allowed a cat, I was obviously a bit put out (until I realised I was a grown adult, and I could do either of these things if I wanted, without having to ask my parents)

Because we're learning on the job?

What seemed very important with DC1, we later realise was totally unimportant.

Because we’ve given up?

I think it’s because the eldest turns out fine so DPs think wow we’re good at this and take it easier with later DCs as they aren’t so anxious.

Because the older siblings knock the parents into shape for the younger ones?

Parents don’t get everything right. We try to ‘do the right thing’ for DC1. It isn’t necessary right, it doesn’t necessarily work. Theory and practice are two different things, we are learning on the job. We get tired. Our priorities change. We recognise our children are individuals. We become more aware that our time with our children is short. We age. We are getting older. Time is precious. Our feelings change.
For example:
DC1 was routine led, sleep trained, organic, puréed home made food.
DC4 was ‘baby-led weaning’, co-slept.
I haven’t ‘gone soft’ I have just adapted to circumstance, age, priorities.
Nothing stays the same.

The only thing that stays the same is being loved completely and unconditionally. That is consistent and constant.