Why is absorption across the lining of the stomach difficult for most chemicals/nutrients?

There are basically two reasons for this phenomenon. First, most substances are not actually broken down fully by the time they hit the stomach. Your saliva begins the breakdown of amylase and other polysaccharides into monosaccharides, but the process isn't complete that early in the GI tract. The pepsin and HCl in your stomach also go to work on the protein matrix in the food, but it's not finished either by the time the chyme (ex-food) leaves the stomach. The lipids are still completely intact. With the protein and carbohydrate matrices still intact, many of the vitamins and minerals are still bound in there, as well. Second reason, there is a layer of mucus secreted by your gastric glands onto the surface of the mucosa (interior lining) that prevents much contact between the bolus of chyme and the actual cells of your body.

I hope this helps.

The powerful muscles in the stomach push the food through the pyloric sphincter. The muscles will not allow absorption of nutrients in the stomach.