Not ALL ancient looms were narrow.
Well, no. Of course not, but wide warps are very rare and noteworthy. Weaving wide warps is expensive and requires more skill and time than narrow ones.
Linen is not stretchy. It poses a lot of problems in weaving; warp threads break easily. A wide loom takes up a lot more space, is harder to put a shuttle through.
Wall looms could be pretty large. Large enough to make sail cloth...
The Shroud was made on a Wall Loom... and it was hank bleached. Soapwort fullered. Hand spun. All things done in the 1st Century and most likely not done in the medieval times when more modern weaving would have been the norm.
In addition there are many extant examples of three over one twill weavings from the period. I don't know where this idea that complex weaving only came about in the medieval period. The Egyptians were fine weavers as were the East Indians, the Chinese, and indeed the people of Palestine area.
There are other fine examples of the weavers art from the 1st Century that are extremely rare... such as examples of the rarest cloth in the world: byssus, which is native to the Eastern Mediterranean that still exist, that have been found in Egypt and Roman burials.