Why HTML is not considered as a programming language ?

Answer 1

Because it merely a *markup language* without any logic.

You can write anything you want and the browser engine will try its best to render the web page without any complain.

Answer 2

HTML means Hyper Text Markup Language

Answer 3

Because it doesnt have a functional purpose like programming languages generally have.
E.g. you cant do maths, there is no expression logic, etc...
This article goes into detail:
https://ischool.syr.edu/infospace/2012/04/05/why-html-is-not-a-programming-language/