Why does it seem like meal replacement and protein shakes don't fill me up or curb my hunger? i normally don't eat a whole lot, but at times can.

One possible reason: There are probably several reasons for protein shakes not curbing hunger. One reason is that protein shakes are a dense food compared to let's say lettuce and spinach. A pound of greens would fill up more of your stomach than a pound of protein shakes. The stomach has stretch receptors that notify the brain when it is filled with food. The brain then generates a feeling of fullness and less hunger