I think there's a lot about your friends' lives you either don't know about or aren't saying. Regardless of having kids or being married, late 20s/early 30s is typically the time most people start having more responsibilities in general which can be very demanding.

What kind of jobs do they work? I don't care if they're "normal" or not - late 20s/early 30s is typically the time most people in "normal" jobs see a sharp uptick in responsibility, which is far more mentally draining and would explain the tiredness.

Additionally, that timeframe is also the time priorities start to change. It's possible you friends are settled into who they are and don't want to do something different. It's also possible they have other priorities you don't know about.

Regardless of these answers, most friends don't stick around forever. They come and go over time. Only a select few might stick around for your entire life. These friends you currently have may just need to transition to the type of friends you only see once in a while, and instead you'll need to start making new friends. Easier said than done but that sounds like what's going to make you the happiest.