Raising kids well is extremely rewarding. Plus its very hard. But those rewards, you can't find them anywhere else. That happiness and fulfillment, dare I say, neither.
I've tried it before having kids - had total blast in European Alps - climbing, alpinism, ski touring, skiing, via ferratas, paragliding, but also ie diving in exotic remote places half around the world. ~5 bigger vacations per year. Traveled all continents except Antarctica backapcking style, ie 6 months in India and Nepal. Life-changing experiences, truly, but that backpacking part was the key to its intensity.
I've met folks who had way more intense lives than mine, since I was still working 100% and did all this during weekends, vacations and after work (or in-between work for long trips). Mostly they were some form of mountain guides who traveled world off-season. But even those folks, when they see me now with what seems like well raised kids so far, damn are they envious. I mean life that you hardly see on some 'influencer' instagram, those folks have no reason to brag about it publicly to compensate for something bad or trying to milk users for money. And most of them ends up in parenthood anyway for better or worse.
All those experiences means very little to nothing compared to experiencing my kids and see them growing up. Yes the hard parts are very hard and you question yourself and your whole life, but the amazing parts... mere words can't describe them. They give life to one's meaning that nothing else does, not even close. There is nobility to take hits for closest ones you love, to endure hardships, it builds and keeps character strong. Overcoming all properly hard challenges does that, do it for 2+ decades and it shows.
I am not claiming everybody should have kids, far from it, maybe not even half of population IMHO (I know, not sustainable, but we have overpopulation now so thats an afterthought). Plenty of broken people around, either inherited mental issues or ones acquired during childhood or even adulthood via traumas or drugs. Some could raise kids somehow but most are bad parents and then it propagates further down the line.