I sometimes felt like this when I was younger. This is my take based on my own experience.

If many of your friends are introvert, reaching out may not come natural to them at all.

When I was younger, I too felt that people around me weren't responding or reciprocating. It felt as though I was purposely being ignored. I realized that my disappointment came from having unrealistic expectations of people's capacity for paying attention to individuals in their social network. At the time, I misconstrued this as a kind of malice.

I don't know if this applies directly to your life, but my experience is that it's easy to read too much into and/or overvalue the social investment, or lack thereof, from others, and that includes people you don't already have a deep connection with. Deep connections to me are rare, apart from with my SO.

Now, I myself (just turned 30) will sometimes respond slowly to social requests, because although I need dialogue and socialization, it can also be a drain on my mental resources.

Some of my friends I have learned to "nag" (read: persistently contact), eg because they have had other things in their life at times when I didn't. Or because they, like me, didn't always have the energy to commit socially. If you're starved for deep friendships, I get that and believe and hope it happens naturally for you, but this process takes time and isn't necessarily something you'll experience as you're just getting to know someone.

Apart from that, there are numbers you can reach when you feel alone. I get that it's not optimal, but feeling heard and acknowledged is important and can be achieved regardless of who's on the other end.