Doubt about AccessExclusiveLock in ALTER TABLE .. SET ( .. );

I'm tweaking some autovacuum settings in a table with high write usage but with ALTER TABLE .. SET ( .. ) this task was impossible, so I did a catalog update  (pg_class) to change reloptions.

Maybe it's a stupid doubt, but why we need to get an AccessExclusiveLock on relation to set reloptions if we just touch in pg_class tuples (RowExclusiveLock) ?