On average, only 14% of those in a couple say they are not very or not at all satisfied with their relationship with their spouse or partner, while 84% say they are somewhat or very satisfied.

And this

The failure rate for first marriage is roughly 48%, 60% for second and 70% for third marriages [source], but at the same time, in 2019 for every 1000 marriages, only 7.6 resulted in divorce, which is the lowest divorce rate in the past 50 years.

So I would not say it is that rare to live happy married life. But it is not like everyone is getting this life. With a hard work I think it is totally possible and is not that rare at all… Although none of those studies give a direct answer on how many happy lifelong relationships there are. One can conclude somewhere between 30-40% of relationships are happy lifelong relationship. And even if this number is lower like 10-20% this is still a very significant number.

When I talk about relationships with my sister she has similar view as you. She can almost never see a truly happy relationship. While I can see it everywhere

It turned out our environments are drastically different. For example she met all of her partners at parties. This is not a general population. While I met my wife in school.

I made most of my friends in school or at work or from being a neighbor (ie owning a house). Most of them are educated and with higher income. If I remember correctly those also have better statistics for relationship success. While my sisters friends are none of that.

I believe it would be wise for you to check if your environment screwed your view too.