pg_attribute getting bloated in size

Hi,

i am faced with bloated in size pg_attribute table, now its 15 GB in size,
from logs autovacuum is showing a lot of now removable rows:
===========>6915 2018-12-14 03:40:02 EET 00000 [7-1]LOG: 00000:
automatic vacuum of table "lb_upr.pg_catalog.pg_attribute": index scans: 1

pages: 0 removed, 1127929 remain

tuples: 169 removed, 14656351 remain, 14501557 are dead but not yet
removable

buffer usage: 1350052 hits, 996326 misses, 20842 dirtied

avg read rate: 19.732 MB/s, avg write rate: 0.413 MB/s

system usage: CPU 5.61s/5.27u sec elapsed 394.47 sec

Why autovacuum cant delete those rows?
What is the main reason that pg_attribute getting bloated?
My autovacuum settings are:

log_autovacuum_min_duration = 0
autovacuum_max_workers = 4
autovacuum_naptime = 51min
autovacuum_vacuum_threshold = 750
autovacuum_analyze_threshold = 5500

autovacuum_vacuum_scale_factor = 0
autovacuum_analyze_scale_factor = 0.2
#autovacuum_freeze_max_age = 200000000
#autovacuum_multixact_freeze_max_age = 400000000
autovacuum_vacuum_cost_delay = 10ms
autovacuum_vacuum_cost_limit = 1500

Will be grateful for any advise!