Ofcourse because it is issued by the national government to be use in our daily needs and transctions. Fiat is the medium of exchange because it is tangible and stable value unlike digital currency. Cryptocurrency will grow year by year but fiat money will remain to be use in our daily lives.
If we don't go outside our home then we don't need fiat, which is impossible since we go outside home, that's how important fiat still is, there are still numerous people who doesn't aware crypto exist.