They generally have RAID boxes with either Ethernet or thunderbolt connection.
Most video editing/rendering is done on big storage arrays anyway. So 256GB storage is more than enough for most serious multimedia professionals.
Besides, perhaps you're the user that needs the multi-TB version. Others, e.g. video professionals wont have huge tens-of-TB source files in their main system drive.
If you add a high performance spinning disk its performance won’t be enough and it will be noisy even when idle.
In the end, it’s better to not add anything and let professionals bring their own storage into the mix.
At the end of the day this won’t be sold to new beginners with no files. The buyers will have their resilient storage and the internal drive won’t be used much anyway.
Unless it’s configured as a developer workstation, but it’s generally MacBook Pro’s role.