pg_am access in simple transaction?

Hi,
I'm doing simple tests with lsof on data catalog with bash script:

#!/bin/bash
for i in {0..2000}
do
psql -U postgres -c '*BEGIN; select pg_sleep(30); COMMIT*'&
done

and i measure number of open files and what files are affected by specific
command.
Lsof has shown me that the only file that was open during this test was:
data/base/13328/2601, which is pg_catalog table pg_am.

Why postgres opens a table that stores information about relation (index)
access methods, while none index evidently is in use?

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