I’ve seen similar support for this feature by literally aligning the text putting any whitespace on one or both sides between the header text and the pipes aligns the text accordingly.
(MarkdownExtra has a special syntax for header alignment which doesn’t map to plain-text conventions, using a colon on the separator line below the header, which is weird.
Taken together, MediaWiki tables look nothing like a table nobody unfamiliar with the syntax would understand what it was trying to do when looking at the source. The syntax in the OP, used by MarkdownExtra, looks like a plain-text table you’d see people manually write in an email or the like, which is precisely the aesthetic that Markdown adheres to and what makes it attractive. Additionally, the use of ! for headers is unusual. Yes, those extra things are the things that don’t look like plain text.