Do you know why certain leading websites such as Vkontakte, Facebook, and Avito, and partly adaptable websites like Auto do not utilize adaptive layouts, despite having sufficient resources and finances to develop a seamless, easily accessible, and adaptable design?
The reason for their decision remains unknown.

Typically, branches such as the mobile version or application are developed by adjacent teams since the primary team is focused on the main functionality/site. This main team is usually occupied with their tasks, indicating job security.

 Hence, it is simpler to employ additional individuals to handle other aspects completely rather than overload the principal team. Consequently, all other teams work independently on their designated areas of the project.

As for me, I often encounter slowness and bugs in my adaptive websites, particularly with large resources like the ones mentioned. In comparison, the mobile version or app is more user-friendly, easily manageable, and requires less maintenance. Consequently, instead of solely focusing on adaptiveness, large websites prioritize what's best for their users.

The explanation is straightforward: bigger projects require more time and resources ($) for tasks such as redesign and adaptation. Adapting to IE8/9, for instance, is not particularly challenging, as the FB audience on these browsers accounts for only 1% of users (or approximately 1 xхx xхx).
Nonetheless, difficulties still arise. Creating mobile versions of services is often a simpler solution so as not to disrupt the entire project.

I have one question regarding adaptive layout of the leading website what are the reason behind it.
software development company

These platforms have been developed over many years, and their architectures are often built on legacy code that can make implementing a new design approach quite challenging. Changing the layout to be fully adaptive would require extensive testing and potentially rewriting large portions of the code, which can be risky and time-consuming.

Another factor could be user demographics and behaviors. Many of these platforms have a significant portion of their user base accessing the sites via desktop. If their analytics show that the majority of users are not accessing the site on mobile devices, they might prioritize enhancing the desktop experience instead. This can lead to a perception that adaptive design is less critical, even if it could improve user experience on mobile.

Additionally, there's the issue of brand identity and consistency. Websites like Facebook have established a strong visual identity that users recognize. A drastic change in layout could confuse users, leading to a potential drop in engagement. They may opt for a more gradual approach to design changes, ensuring that any updates align with their brand image and user expectations.

Moreover, performance is a crucial consideration. Adaptive layouts can potentially lead to longer load times if not optimized correctly. Websites with heavy traffic must prioritize speed and efficiency. If a fully adaptive layout compromises these factors, they might choose to stick with their current design.
There's a matter of experimentation. These platforms often run A/B tests to gauge user response to different layouts and features. If the results from these tests indicate that users prefer the existing layout or if the new designs do not yield significantly better engagement metrics, they might decide against implementing adaptive layouts.

Because responsive design provides more flexibility across multiple devices, ensuring a seamless user experience, many well-known websites choose responsive layouts over adaptive ones. Predefined screen sizes are necessary for adaptive layouts, which may reduce their adaptability for more recent devices. Because responsive design adjusts dynamically to various screen resolutions, it also makes maintenance easier.