Cotton is cheap and easy to clean (which is pretty much why your using a liner in the first place), but does like moisture, which is why cheap sleeping bags have a cotton inner (they say its for comfort, but we all know because its cheap). So they can be sweaty.
Silk 'breathes' a lot better, and is often more comfortable. It also squishes down smaller when packed. I have to admit that I also find silk a little 'cold', but thats just me. Silk is actually really good at either cooling you down/keeping you warm (think silk shirt in summer, pilots with silk scarves). There are also synthetic ones, although the fleecy ones are more designed for an extra bit of warmth.
Remember, your comparing cheap cotton on Ebay (or relatively cheap silk on Ebay) with a Rab high quality silk bag (designed to fit their bags) in a shop - you might be paying over the odds, or you might be buying cheap, and buying twice. You pays your money...
They dont care about the material, they just care about feeding on you, unfortunately. You can certainly use a liner/travel sheet in hostels etc to keep off dirty sheets/ help stop beg bugs (any barrier is a help), and Sea to Summit/Lifeventure do ones which are impregnated with stuff to keep all sort of insects at bay
https://www.rei.com/learn/expert-advice/sleeping-bag-liners.html &
http://www.independent.co.uk/extras...vity/5-best-sleeping-bag-liners-10082198.html. If your out in the countryside in a tent, I wouldn't bother.
We were hit some years ago by three infestations in 4 years, caused by neighbours, so I got to know about them (and paranoid) very quickly. Bed begs feed on blood, and seem to home in on their victims using our exhaled CO2 to guide them. 40% of people dont react to a bite (I'm not sure I was even bitten, but I certainly didn't react), and its easy to think the bites are mozzies/chicken pox at first (thats what we thought the kids had). Horrible creatures, and total nightmares to get rid of. If you do have a problem with them, while you can call a professional, it wont be cheap and might not be quick. These guys are really good at supplying the proper stuff you need:
https://www.pestcontroldirect.co.uk/acatalog/Pest_Control_Direct_Bed_Bugs_28.html , and you can do the job yourself.
Now trying not to shudder at the memory!