Why should we use hexadecimal values for colors instead of directly giving their names?

Answer 1

Because names are limited to around 140, while there're millions of colors.

Answer 2

As you may know there are multiple shades of many colours, light red, dark red, maroon red, chestnut red and I can go on forever, if you give just their names you'll never have access to perfectly matching colours together on your website since your range of colours will be millions of times less.

also hex allows you to easily modify a colour at any time to match more with your website, you can make the colour cooler or warmer, add in more blue, green or anything to get that perfect match.