When someone gets a new romantic partner, they usually take priority for a few months. Once the novelty wears off, they re-invest in their friendships again.
I think it’s just part of life and a lesson to learn, that many learn the hard way, not to get too caught up in a new romance and drop your friends. But, like most things, it’s easier said than done (especially when you’re young!).
You will probably make the same mistake OP when the time comes. I always found that the “I would never ditch my friends for a man” girls were the first to disappear the second a man looked at them.
Once you’re out of that first relationships stage, when people marry etc or are in a committed relationship, their partner becomes their family and will often ultimately be number one.
Ditching all your friends for a guy with a motorbike when you’re 17? Not very wise. Prioritising your long-term partner and life together over a friend? That’s just life!
I don’t think there’s a hard and fast rule of who should “always” be prioritised. A friend’s big birthday would come before a random date night with my partner for me, but similarly, if I had plans in advance with my partner to do something, I wouldn’t be free for a last minute girls night.
I think everyone finds the balance that’s right for them, and I don’t think you can begrudge anyone their choice on that front OP. It sounds like you are single and therefore prioritise friendships. This is likely largely based on your current circumstances. When you’re more to juggle, somethings gotta give.
Also, sometimes friends have a different view of the friendship, and it maybe just isn’t as important to them.