Why do guys I talk to always do this

It depends what the objective of talking is.
If it’s meant to possibly be romantic then talking for weeks without any offer of meeting up is inevitably going to go dry.

I’m not going to shag someone off the internet to thank them for talking to me for a week. Theobjective is that I’m lonely and want someone to talk to

But you aren't talking, you're messaging, and that gets boring very quickly. There's no real connection or nuance.
Get some hobbies, play sport, do evening classes, volunteer: all ways of meeting like-minded people to befriend.

I’m not going to shag someone off the internet to thank them for talking to me for a week. Theobjective is that I’m lonely and want someone to talk to

Every time I start talking to a guy, we have good convos for like a few weeks to a month then he starts taking longer and longer to reply until I get like one message every three days and I usually just don’t bother to reply after that. I’ve always struggled to connect with people and felt that I give off a “weird vibe” or aura that other people instantly pick up and make them not like me. And that’s the reason I’ve enver had a friend or relationship. So I mainly try to find people to talk to online and then that always happens. Like even online people pick up on this weird vibe. Or is it all that guys my age (I’m 20f) are porn addicts and get bored because I’m not hyper sexual and don’t want to talk about sex

Pretty much that's what it js