Imagine you attended a meetup for a new hobby.
You were introduced to people, but most of the regulars only spoke to other regulars. Imagine if you weren't included in any activities they were organizing. You probably wouldn't stay involved in that group (or activity) for long.
The group had an impermeable boundary. Newcomers couldn't feel like one of the group. Newcomers couldn't see an easy path to become an accepted member of the community.
People participate in communities to satisfy their social needs more than their informational needs. Those social needs include being an accepted member of the group. It includes having friends they've made through that activity who they want to check in with frequently.
If the boundary between registering as a member and being an accepted member is impermeable (or seems so) most people will give up. They will leave the community.
Community managers that overly serve the veterans do so at the expense of everyone else. You serve the veterans and they feel they're part of an insider club. They begin to act that way. Community veterans need less of your time than newcomers.
The solution is to create clear paths to becoming regular members of the community (profile community members and their journey through the community, for example), showcase the contributions of newcomers as well as regulars, and directly address the issue with veterans if it arises.
p.s. Final day to register for FeverBee Professional Community Management Course.