I'm honestly not sure how much of that would be related to the query planner vs linked server network code.
Speaking of which, linked servers (cross OS, cross SQL type database queries) are native in MSSQL, but require a third party odbclink in postgres, which does not appear to have been updated since 2010[0].
Linked servers in Postgres can be implemented using foreign data wrappers (part of the SQL/MED implementation in Postgres - MED = management of external data which is actually in the SQL spec):
https://wiki.postgresql.org/wiki/Foreign_data_wrappers
but stuff like "Does not compile with PostgreSQL >= 9.2!", "A patched, but completely untested version", and "The current version does not yet support WHERE clause push-down, column push-down, JOIN push-down, or write operations."[0] are riskier (and a harder sell to management) than having the functionality backed in the core engine and admin interfaces.
Speaking of which, linked servers (cross OS, cross SQL type database queries) are native in MSSQL, but require a third party odbclink in postgres, which does not appear to have been updated since 2010[0].
[0] - http://www.cybertec.at/en/media/downloads/