Why do you think high threadcounts are a marketing gimmick in this context? Threadcount is just a measure of the number of threads in a square inch of fabric. The higher the threadcount, the denser (and heavier) the fabric is. Higher threadcounts also mean that a fabric is less breathable as less air can pass through.

Generally, craft stores sell only a handful of different threadcounts of a given type of fabric, usually very low threadcount, but in dozens of different patterns, because customers are buying based on patterns, not fabric density. Also, higher threadcount fabrics are more expensive and thus less in demand, so generally need to be purchased special order. Alternatively, you can buy directly from a clothing/fabric wholesaler (though the wholesaler may ask for a reseller's license first).