The reason is that all natural fibres are fiendishly complex in structure. We’re probably closer to replicating a sheep from raw atoms than replicating wool.
This is what you’d have to recreate in your lab. Good luck with that.
It’s not much simpler to replicate a plant fibre like linen:
There are, however, innumerable artificial fibres, mostly derived from petrochemicals, which mimic the qualities of natural fibres. Most can only imitate the feel of the natural fibre. For example, acrylic fibre has the softness of wool – but not the warmth, rainproofing, elasticity, inflammability, or biodegradability; and acrylic has many disadvantages besides, such as being a significant source of microplastic pollution in fresh and saltwater.
It is possible to make something that looks like wool, either with plant materials or, more usually, petrochemicals.
Cotton and linen, two common plant-based yarns, can look the same but are cooling rather than insulating, flammable rather than inflammable, heavy rather than light, and liable to stretch irreparably rather than keep its shape. Other plant-based fibres such as viscose and ramie (e.g., bamboo, sugarcane, cornsilk) are similar. Their production, and that of cotton in particular, is also rather bad for the environment.
Petrochemicals, such as acrylic, are sweaty, very flammable (and melt into your skin when they do, which is oodles of fun at the Burns Unit), appallingly bad for the environment for centuries, and are only “easy care” if you wash them on the same cycle as wool – and even then, they shed microplastics into the watercourse and thence to rivers, lakes and seas.
What kind of “knitted yarn” are you talking about? Both wool and polyester are yarns that can be knitted to make sweaters. Polyester is a lousy yarn for knitting: the fabric knitted from it is not warm in cold weather, and in warmer weather, it quickly becomes sweaty and damp. Like its synthetic counterparts, it is dirt cheap, which is why virtually every cheap sweater you see in the shops is made of some kind of man-made fibre.
I’d suggest you join Ravelry, and spend 6 months or so in the Yarns section, studying all the different fibres, constructions, weights, blends, and drafting methods and how these influence each yarn’s qualities, dyeability, fulling/growth, draping, etc., etc. Spend some time learning from others on the Yarn and Fibre Forum, even ask a few questions. Discover and follow Rav yarnies like Clara Parkes. Take in a handspinning class. KNIT SOMETHING.