As somebody who works in tech, you'd be amazed at the difference between internal and external websites.

Internal here refers to company only websites, only accessible to employees while connected to their company intranet, with the exception of sites used to register software tokens used for VPN access.

External refers to websites accessible on the Internet at large, intended for customers.

Internal websites are often designed to perform a function and perform it well. Anecdotally, I've seen internal websites that were designed in the Geocities days and it shows. They also require Internet Explorer to function properly in many cases.

External websites are designed for function, performance, while also looking professional (This definition changes weekly/monthly in many cases).

There are some companies that pay UX designers to clean up the looks of internal websites (or transition them to modern platforms) but there are many more that do not.

To me, it makes sense that an admin panel might not see major changes from the early days of Twitter (except possibly the addition of those buttons which would be used to control content, if they do indeed exist).

Why change something people are already used to if it will require retraining and time to master its use? From a profitability standpoint, it makes more sense not to change it.