Because you don't in a Merlin going to windward. Initially you pick up speed proportional to the true wind so there is no apparent shift if the wind speed increases slowly. The Merlin in displacement mode has a maximum speed of just over 5 knots and will sort of plane to windward getting up to a bit over 6 knots I think as measured on GPS. This isn't enough to push apparent wind forward that much if the true wind is say 12 knots (look at the vectors). Even so you are sailing to a wind that is much closer than the true wind, and there is also an initial freeing and subsequent apparent heading of the wind in a gust. Where it makes a big difference though is downwind where the apparent wind will pull forwards significantly on a broad reach even in a Merlin. Of course in some boats (catamarans, foilers) the apparent wind is always well forward of the beam as if you are doing 20 knots on a beam reach in 20 knots of breeze you will feel ~28 knots of breeze at 45 degrees. Also look at the mainsail sheeting angle on 18ft skiffs going as far downwind as they can - still sheeted in fairly hard with the "spinnaker" more like a large genoa in cut as apparent wind is still well forward. Eric Twiname's book Start to Win (from the 1970's I think) has some excellent explanations as well as a lot of pictures from the Welsh Harp.
After some re reading I find it will be illegal. Why do 40ish people get to have there rudders made legal when they failed to read the rules? Yes I can modify it, but that's besides the point. I shouldn't have to. Anyway, here's a clearer video. https://photos.app.goo.gl/UQbmTQYQ2s3x3jhn7 Chris I appreciate it's hard, but that's the road that's been chosen, to modify rules through the rya. I won't get the files and epoxy out just yet...
Interesting point Colin. I think Merlins do point pretty high and skiffs etc. planing to windward a bit lower. The only boats pointing significantly higher in Eric Twiname's book are slim deep draught keelboats. Thing is we are so seldom on the same racecourse and even when we are (e.g. Bloody Mary) the foilers are going so much faster it's impossible to make the comparison. Rigs are presumably designed to work very close to the wind and the drag component which is so prominent looking at the physics of Merlin sailing to windward is very small indeed.