The keyboard flex is probably there of how it's build. (I'll compare with the macbook pro line-up) Unibody construction like it says is the body of a computer made from one piece of material, in this case Aluminum. The difficult part of this type of construction is deciding where the point of entry is if you want to put something in it. If you want the product to be priced reasonably you keep the point of entry big, like the bottom panel of the macbook pro.
The reason the macbook pro has practically no keyboard flex is because the keyboard is not the point of entry to the unibody, and it is on the opposite side of the bottom panel, it has all the strength of the unibody. To clarify, the top side of the macbook pro's, the side where the keyboard and trackpad are, needs to be as sturdy, if not sturdier than if the bottom side wasn't the point of entry, the point of entry is on a side. It also needs to have the same strength as if the bottom panel wasn't there.
So to get on the reason of why there's keyboard flex on the surface laptop, is if you look at pictures of the underside of the laptop, you see no screws, no lines (gap between panel and unibody). As I told earlier, to keep costs down, the point of entry needs to be big, the sides are too small for an efficient way to mass produce these things. So the only point of entry is the up-side, or the side where the trackpad and yes the keyboard is.
To compare to the macbook pro again, the design there needs to be as thin as possible, and so the bottom panel, if you ever held a macbook pro, you know it flexes in the middle a little bit. And because the point of entry is probably the up-side of the surface laptop, so it's comparable with the bottom panel of a macbook pro, the keyboard has flex.
To reason further why there's flex: microsoft could make a keyboard for the surface laptop but that would mean the laptop overall would need to be thicker, since then the keyboard and trackpad panel needs to be thicker to prevent the flex. Of what I see is that they did this to an extend.
I guess the creators didn't want their already thick device be even more thicker than the competition e.g. the dell XPS13 also the price would increase and the material production cost for the heavier and thicker panel too, so that would mean less profits.
To conclude, the keyboard flex is there because it is the point of entry to the unibody. It doesn't have the strength as the back of the surface laptop nor does it need to or the laptop would be thicker.
Yen Aarts 03-06-2017