Whilst iOS/iPadOS will show individual file sizes, the OS does not natively expose folder sizes. Contributors here have no knowledge of Apple’s feature or coding design decisions - and cannot provide authoritative comment on Apple’s reasoning behind those decisions.
That said, just because native tools do not provide the functionality that you seek, there is a rich ecosystem of third-party Apps that extend natively exposed OS functionality. Whilst this may require purchase of a third-party App, most are not overly expensive for the additional functionality that they add to the system.
Whilst freely acknowledging your desire to find additional functionality for no cost, you are going to have to use a third-party File Manager App. You’ll find many in the App Store - but a few have good reputation and following.
File Browser - Document Manager (recommended for your stated needs):
https://apps.apple.com/gb/app/filebrowser-document-manager/id364738545
FE File Explorer: File Manager:
https://apps.apple.com/gb/app/fe-file-explorer-file-manager/id510282524
Both of the above have the equivalent “Pro” versions. If considering either, you’d be well advised to check which version is more suitable for your needs before purchasing.
Returning to native functionality, Apple do invite constructive feedback and feature requests via their product feedback pages:
Feedback - iPad - Apple
I hope this information and insight proves to be helpful in resolving your immediate issue.