correlated query as a column and where clause

Hello All,

I am wondering,  why I can not add the following  '  A > 10'  in the where  clause  i.e.   'where nspname !~* 'pg_'  and A > 10'

Select nspname, (SELECT count(*) as count FROM pg_tables where schemaname = nspname) as A
FROM pg_namespace
where nspname !~* 'pg_'


Thanks in advance