I dont think its always the case
As a teen and now as an adult my mum doesnt know me the best. I also think my mum didnt parent me in the best way for me, but what she thought would have worked for her if that makes sense? Where she was preoccupied by her own thoughts and experiences, she didnt see that actually there was other much better ways of making me learn, soothing me etc.
Sometimes we are too close. My friend has a 4yr old who has shown clear signs of asd since he was a baby for example hand flapping. As friends and family we noticed this but personally i never brought it up because I assumed she'd be seeing what I see and thus it didnt seem any point of mentioning it. It turns out that she hadnt, and was very shocked when his nursery raised the signs they had noticed. I think because she saw him everyday she had been too close to see what other people saw quite easily. Once that conversation had been had, she suddenly saw all the things shed been missing
Sometimes parents arent up to date on guidance etc and i tend to hear the phrase "i know them best" as an excuse to not follow safe sleep guidance or do other risky things. I think we should be able to challenge mothers when they are putting their child at risk.
Ive also had two occasions where ive pointed out something to a friend who wasnt aware of it. One was that their baby was way too big for the style of car seat, the other was feeding baby rice and wasnt aware the guidelines dont support it as was just following what was the norm when she was little. As it was sensitively done, and hopefully respectfully theyve both thanked me.
As a parent i find it hard when theres an expectation that i will know everything when theres so much to know. Im happy to be treated as the expert for things like what food she eats or how best to settle her but i dont think that should mean i dont listen to advice. Its been helpful when people have said hey why dont you burp her like that etc. Sometimes im so in the middle of things its hard to see things to change.
My mil suggested some changes to bedtime routine, that 100% worked. We were too deep in the sleep battle to step back and see different ways of doing something
Generally people all have different techniques of doing stuff, and different parenting styles. Me and DP both sometimes think our way is better and its sometimes painful to watch eachother try and manage a five year old tantrum for example but i think ultimately it will be good for our kid to have two different parents.