I've written a lot on this, it's pretty fascinating to me, and there's multiple reasons:
1: looking into a lot of people's phones is just pure chaos, there's dozens and dozens of unread messages on three or 6 different apps, group chats, work emails, Snaps, DM's, on and on, it's overwhelming, and people tend to "triage" as in respond to only what the feel is the most important, and as part of that, they may feel like replying a few days later is too "awkward", as a lot of texts are time sensitive to a degree, people realize this, even if they don't verbalize it. We used to kind of think "we all have our phones 24 hours", and that's true, but many have lost control in a sense
2: Ascribing a lot of meaning to digital communication, through gesturing or signaling, even the words you use, we use, "ghost", "left on read", we are Anthropomorphizing and ascribing meaning to behavior that isn't always that meaningful, someone didn't respond to your message, but if that person walks up to you right now, what is your interaction going to be? Probably fine. Maybe even intimate to a degree.
First example, you text someone who likes you, (Hey, are you free tonight?") they are, but don't know if they are done w/ their ex, so they say nothing, even if you will 100% see this person again. They were uncomfortable. They chose to avoid.
-second example, people use the phrase "talking phase", to describe essentially texting with someone, but the meaning is romantic/sexual, therefore that bleeds into all other communications
-As in, if you are "talking" with someone, there is some deeper meaning to that, or is that "cheating", is that "showing interest", and so on
I collect these stories all day, you can't take it too personally, or like a full on rejection, One of the harder parts though, is that you know what it feels like when people are texting "right", everyone knows how to do it, quick responses, building to something. But, that doesn't always happen, so it gives the feeling of rejection.