My question for @Emmad-Kareem would be: what other apps that you use do provide such a feature?
Have you asked the Excel developers to allow you to choose what icon you associate with spreadsheets? Do you expect IE/Edge/Chrome/Firefox/Opera to give you the option to choose a different icon for html files on your computer? Do you expect [insert your music-listening program here] to allow you change the icon for music files associated with that program?
I have wracked my brain over all the different types of files I regularly use with default associations, and cannot come up with any type where my preferred application allows me to choose any icon I want for files they handle. If you know of any, I would be curious.
Instead, in my experience, the general philosophy is, “if you want our program to be the default for a filetype: great, we’ll automatically choose an icon for you. If you don’t like that icon, it’s up to you, using the already-existing registry tools built into the OS (or any other third-party registry-editing tool you so desire) to do the change; you’re allowed to do it, but we’re not going to spend our time to do it for you”.
(re: third party registry tools: nirsoft has the FileTypesManager, which can be used to manage all sorts of things to do with the verbs/actions and icons and programs associated with various file types / extensions)