Postgres index on timestamp column not utilized

I have a database with a column called timestamp of type timestamp without timezone, there is a btree index on this column.

I have a query as below

SELECT
   *
FROM
  employee 
WHERE 
  timestamp >= timestamp '2020-01-27 13:24:09'

However, the explain analyze shows that the index on the column timestamp is not used:

Seq Scan on employee  (cost=0.00..5498.73 rows=34377 width=381) (actual time=0.016..37.944 rows=34251 loops=1)
   Filter: ("timestamp" >= '2020-01-27 13:24:09'::timestamp without time zone)
   Rows Removed by Filter: 21167
 Planning Time: 0.255 ms
 Execution Time: 40.277 ms

If i change the query with filter condition including a timestamp which is 1 month instead of 1 year (as above) then the index on the column is used.

Bitmap Heap Scan on employee  (cost=59.51..4510.25 rows=2996 width=381) (actual time=2.164..5.204 rows=2958 loops=1)
   Recheck Cond: ("timestamp" >= '2020-12-27 13:24:09'::timestamp without time zone)
   Heap Blocks: exact=208
   ->  Bitmap Index Scan on timestamp_salary_idx  (cost=0.00..58.76 rows=2996 width=0) (actual time=2.113..2.114 rows=2958 loops=1)
         Index Cond: ("timestamp" >= '2020-12-27 13:24:09'::timestamp without time zone)
 Planning Time: 0.195 ms
 Execution Time: 5.485 ms

Why is the index not used for queries with a range past 1 year?