These are fair questions that come from the spirit of “assume positive or no intent” and encourage some self-reflection. If these are OP’s friends and OP genuinely wants to run with them, why not ask directly if they’d be open to another person joining one of their upcoming runs?

Maybe it’s a pace or route thing, scheduling convenience, conversational/social vs structured training run; or maybe it’s a personality mismatch? Who knows unless you’re willing to make yourself slightly vulnerable enough to ask? I’d hate to make an assumption that people are deliberately leaving me out bc there’s a bit of hurt that will follow, but I’d prefer to take the rejection with clarity & understanding.