Why is double plate necessary in this case?
The original framing here is balloon framing.  It doesn't work the same way that platform framing works.  (Platform framing variants are almost universal for well over 50 years in the US.)  The studs are continuous from the plate under the rafters to the sill on the foundation.
It replaced post and beam construction because it didn't rely on massive straight timbers for posts, beams, and girts.  It went out of style when it got to be hard to get lumber long enough for those long studs.

The board you're calling a header isn't really a header, it's a ribband or ledger.  The joists rest on the ribband, but are also face nailed to the studs, so the ribband doesn't do all of the transfer of the floor load to the studs.  The ribband doesn't span across openings the way a header or joist or beam does.
I don't have any technical understanding how window openings in balloon framing worked.  If you're used to platform framing headers, some of what you see in balloon framing looks like it wouldn't work.  But it has held up very well in homes built from the late 1800s to maybe 1930.  The walls are surprisingly strong.  They usually just have a single horizontal 2x4 spanning the small openings, maybe a doubled 2x4.  (Of course in those days, a 2x4 was really 2&quot; x 4&quot;, and the quality of lumber in general was far better.)  I have seen pictures of much more elaborate methods, but haven't actually seen them in the wild.  I suspect they were used in the load bearing walls on bigger openings in high end homes.

I suspect it has something to do with balloon framing walls working as a structural assembly.  With the sheathing and the ribband, the headers don't support the entire weight above the window.  Since the studs are continuous, there are no cripples above the header, the studs above the header continue all the way up.  In a sense the headers they used, if you can even call them headers, hang from the studs above.
I am just guessing, but I don't think architects and engineers have a super detailed understanding of how balloon framing works like they do with platform framing.  Maybe your architect figured it was better to just overbuild it rather than try to figure out what's the minimum you can get by with.  They probably did you a favor - they'd have charged you more for the detailed analysis than you'd ever recover in the cost of lumber you save.