And here's why:

It is possible to collect and weave spider silk into garments, but it takes an awfuly long time and I'm not convinced that it's at all possible to dye the silk. Still, the time cost in making acromatula silk cloth is not the part with which I have issue. In fact, the size of individual acromantula would make the collection part much easier, and therein lies my problem.

How much larger than even the biggest species of ordinary spider is an acromantula? By comparison to a Goliath Birdeater, an acromantula is vast. Titanic. It's silk threads must, therefore, be proportionally larger, too. Now, I realise that spiders are capable of spinning threads of different thicknesses - and other properties - for different purposes, but I still cannot believe that even the thinnest of these is less than a millimeter thick, and that's being generous. I imagine them to be more like wool yarn.

So, far from a fine and smooth fabric, acromantula silk would look - at the most generous - like rough hessian or coarse linen. But probably more like knitted wool. And that's saying nothing about how a fibre as tough and strong as spider silk would actually feel to wear. How do fabrics made of the toughest and strongest fibres we know feel? I'm thinking nylon webbing for seatbelts or straps, or kevlar. It's rough stuff, and certainly not something I'd want to wear against my skin.

Now, don't get me wrong, acromantula silk would be a status symbol; the effort and risk involved in harvesting silk from a man-eating and sapient species of giant spider would make the material incredibly expensive to obtain, and it would have some remarkable protective properties. But it would not look like the finest silkmoth silk. It would not flow like water, nor shimmer. It would not make elagent garments. Ankle-length robes of the stuff would look like chunky knitwear, and be stiff and unpleasant to touch.