From the first look at the system tables and their relations the reason of this limit is related to the datatype SHORT of column RDB$RELATION_ID of table RDB$RELATIONS and other related columns from tables ("RDB$DATABASE", "RDB$FORMATS", "RDB$PAGES"). Also I noticed that relations between different internal DB objects (such as columns, tables, indexes) are based on the "Table Name" (RDB$RELATION_NAME), but not "Table ID" (RDB$RELATION_ID). Looks like the primary used of RELATION_ID is to identify the PAGE in the data file.
Consider these thoughts it seems to be not such a big and complex task to change SHORT datatype to for example LONG for RELATION_ID column (and related to it source code) to increase the maximum number of tables.