Why does a shofar have to be from a from a horn whose inside layer can be removed?

The symbolism of וּמַלְתֶּם אֵת עָרְלַת לְבַבְכֶם , removing the layer of "foreskin" on the "heart." A difficult task indeed.https://thechesedfund.com/rabbikatz/support-rabbi-katzz-podcast