A couple of issues with little to no distinction:

• Community rules. If 4 communities were in the same thread at once this would be harder for moderators to moderate, and for users to follow the rules/report. Since they can’t tell easily from which thread this comment comes from

• Threads are not always crossposted with the same context. Example:

• lemmy.dbzer0.com/post/40368868

• reddthat.com/post/37396001

These threads are posted in different communities, their comment sections would be very different