Why does <thead> get stripped out of tables when importing via rest API call?

[Walter Sobchak]

Our product has a Confluence import function. We use the following API call:

/rest/api/content/5275684?expand=body.view

to pull the HTML content of a page with the ID 5275684

When you inspect a page on Confluence, you see a <thead> tag within tables. However, when you run the above API call, the <thead> does not appear (i.e. when the page is imported, the <thead> gets stripped out).

Can anyone please explain why this is the case? Thank you!