Because new HDTV do not have an internal RF antenna, and second RF signal (analogue) have been discontinued in the usa for a number of years.
You need a external HDTV antenna (looks like example below) and use this web site to see what channel will be available in your region.
https/www.tablotv.com/tools/ then pick an antenna that re3flects the range of the channels will be at (if you pick a 20 Miles antenna and the tv channel is considered 30 miles out you will not see it. this is important)
Also look for top 5 HDTV Antennas web sites, and have to consider is your TV on an external wall, or internal wall of the house, and in what direction (N.S,E,W) it is pointed, as this will also affect reception. ref: https/www.digitaltrends.com/home-theater/how-to-buy-and-install-an-hd-antenna/