The reason is quite simple, AWS is still the leader in the public cloud market and we have thousands of members that are looking for training to pass the certifications (actually we have a dedicated product called CloudCareer for AWS for that).
You're right, they are changing very fast but that's part of the tech world and definitely something that we have already saw for other platforms.
Think about all the companies like Trendmicro or Reverbed that are building product for AWS or Google or Azure, on top of these platforms. Having an AWS certification is really the first goal there and the first need for millions of companies. The same is going to happen with Google Cloud, Azure and so on.
For CloudAcademy.com we have a great challenge about this point: we are developing a technology to keep our content automatically updated with the help of the community and of our algorithms.
Stefano