Why companies does not provide stock android to their phones?

Everyone on the earth want their phones to be secure and adfree, there is no person who wants to see ads on his phone everytime he unlocks. So if the demand is so high then why do you think companies do not provide the stock android even to their high end  flagship phones. 

The answer is simple stock android are not as good as the default applied skin on android many companies provides. They are quite simple but not everyone wants a simple phone which they have paid a thousand dollars. 

Why smartphone companies does not provide stock androids?

Lack of functionality

There are many features which any specific companies have better than the stock android in their applied skin like MIUI has the control panel which provide an easy interface for your regular tasks, and samsung has their built in wifi calling feature in their one OS. The gestures are more efficient some company's UI (user interface) like in oxygen OS of oneplus. The always on display have more vibrant features in samsung's one UI, In some phones there are game modes in their UI, There is the feature to run multi user account in the single phone, The facility of dual apps. These features are not available in the stock android.

Dull appearance of stock android.

Excess of anything is bad. The stock android sometimes sucked in its own simplicity, It has such a simple user interface as sometime its feels boring to use it. On the other hands companies like oneplus and Xiaomi is providing many flashy features in their user interface to enhance the looks of their smartphones like dynamic wallpaper in their phones, customizable themes, better volume sliders, enhances animations. The customizability of the stock android is not as extensible as these companies provide in their own android based OS. 

The modifications.

Some of the phones comes with some of the inbuilt modifications for which they have to make their whole interface, the best example is the gaming and foldable phones.

The identity.

Most of the companies are identified by their UI's, most people buy there phones just because they like their interface. Like samsung's one UI is with samsung since the beginning of the era when samsung started making android phones. The UI's are not just a skin, for companies they are their identity their way to be identified. They have spent years and years of research and development to improve their interface, then how they will simply switch to stock one.

To look different form the competition.

You if you have spent a thousand dollar on a smartphone and internally it looks same as the 300 dollar smartphone will you regret your decision for your purchase? I don't know about you but I'll definitely will! So this is a marketing gimmick to make their device feel more valuable by reducing the no. of facility on their older ones.