Because one is a query that counts all the rows and the other is a command that retrieves stats the DB engine maintains about the table. There isn't any firm guarantee that the table statistic will be up to date so the only way to get an accurate count is to count the rows, but it might be that you don't need it to be perfectly accurate all the time. You can thus choose either, depending on your desire for accuracy vs speed etc.

See here this screenshot from https://pingcap.com/docs/stable/sql-statements/sql-statement-show-table-status/

You can see the example inserts 5 rows but the table stats are out of date and the table still reports 0 rows. Running ANALYZE TABLE will (probably) take longer than counting the rows, but the stats will be up to date (for a while at least) afterwards.

A suggested approach to get a reasonably accurate count of the table size, when SELECT COUNT(*) is taking a long time to complete, could be:
ANALYZE TABLE my_table_name; SHOW TABLE STATUS LIKE 'my_table_name';

This comes in especially handy when importing a large amount of data into a table, and you want to track the progress of the import process.