Moses explained why Pharaoh’s request could not work. He said it would offend the Egyptians. The Egyptians looked down on the Hebrews and other Semitic peoples. They considered them uncultured and unclean (see Genesis 43:32).
The Hebrews were leaving Egypt in order to worship the Lord (see, for example, Exodus 5:1; 7:16; 8:1, 20). God’s purpose in the exodus was not only freedom from slavery. He wanted to bring his people into a right relationship with him.